Today, we will look at the newest contender in the Sennheiser Six Hundred series headphones, the HD 660s2 vs. the legendary HD 650 headphones!  This was an article many of you asked for, and we are happy to share my thoughts on the two headphones and who each headphone might be a better fit for.  This article will primarily discuss my impressions of the headphones regarding build quality, comfort, design, and sound quality.  

Sennheiser headphones, especially the HD 600 and HD 650, have likely been in the homes of more audiophiles, audio enthusiasts, and music lovers worldwide than any other audiophile headphones. 

Build Quality, Design, and Comfort: Examining HD 660s2 and HD 650

So, let’s start with both headphones' build quality, design, and comfort before we get into my sound impressions.  Both headphones are exceptionally well built of hard plastic material with padded headbands and soft ear pads.  The clamping force is pretty close; however, I would say the review pair of HD 650 Sennheiser sent me have slightly less clamping force than 660s2.

This could be that the HD 650 is slightly more worn in, or there may be marginally less clamp in the build of the HD 650 vs. the 660s2.  Both headphones are comfortable, and the clamping force can be improved by allowing them to stretch a bit through regular use or placing them on a foam block. 

Impedance, SPL, and Frequency Response

Both headphones have an impedance of 300 ohms, making them fantastic for use with tube amplifiers. According to Sennheiser, the 660s2 has an SPL of 104 dB with a THD of less than 0.04% and an FR of 8-41,500 Hz.  The HD 650 has an SPL of 103 dB with a THD of less than 0.05% and an FR of 10 Hz-41,000 Hz.

Pricing and Accessories: HD 660s2 vs. HD 650

Both are over-ear open-back dynamic headphones weighing around 260 grams and assembled in Sennheiser’s Ireland factory.  The normal MSRP for the HD 660s2 is $599.95 vs. the $549.95 asking price of the HD 650.

The 660s2 includes a 5.9ft  ¼” cable with a 6.3mm to 3.5 mm adapter and a balanced 4.4mm cable, while the HD 650 includes a 9.8ft long ¼” cable with a 6.3mm to 3.5 mm adapter. 

As far as looks go, I think they are both decent-looking headphones, with the matte black and copper accents of the HD 660s2 being my preference vs. the shiny grey finish of the HD 650; however, both headphones look good, and this will come down to your personal preference.  

Sound Impressions: Comparing the Performance of HD 660s2 and HD 650

So let’s get right to it, how do they sound compared to each other?  For my sound impressions, I used a few different amplifiers and DACS to formulate my take on each of the headphones and how they compare audibly.  

My reference Tube Amplifiers for this comparison article are the Woo Audio WA2 OTL Amplifier and the McIntosh MHA200 Headphone Tube amp.  My Digital analog converters are the Chord Hugo TT2 feeding the MHA200 and the Chord Hugo 2 feeding the WA2 OTL amplifier.  My solid-state reference headphone amplifier for this article is my Chord Hugo TT2.  

I listened via Qobuz and Tidal via Roon to several playlists I use to test gear and will share some of my thoughts on how the two headphones compare audibly, referencing a couple of the music tracks from my playlists.  

If you are a measurement lover, there are measurements for both headphones that can be found online, with HeadFi and Jude’s measurements being my top choice for zero bias and one of the most accurate rigs for headphone measurements.

I enjoy checking out measurements as a reference but have also learned that measurements don’t always translate to what I hear when auditioning speakers or headphones at home.

Suppose you can’t make it down to our Southern California 10,000 sqft showroom. In that case, I encourage you to listen to any headphones you are considering purchasing yourself to formulate your thoughts and opinions as we all hear a bit differently, may have different audio rigs, and one person’s opinion is just that.  

For instance, many people love the HD 600 based on what many refer to as the most accurate Sennheiser Six Hundred series headphone tuning, which pairs best with the genres of music they most often listen to.  

They are a great set of headphones for an incredible price, especially when paired with the proper tube amp.  However, for my taste, the HD 650 was my first love and has remained one of my favorite headphones. I prefer the OG 660s and the new 660s2over the HD 600.  

While the HD 600 sound is more detailed and neutral with greater treble emphasis and air, they also sound thin and less romantic to me than the warmer or darker-tuned HD 650.  This doesn’t mean your love for the HD 600 or my preference towards the 650 over the 600 is right or wrong, just different, which is why we always encourage you to listen for yourself.  

I want you to consider me your brother from another mother, a music lover first and foremost, and a passionate movie enthusiast. While I may reference technology, product design, and specifications, I won’t likely go down any measurement rabbit holes and leave that to others that enjoy that.

I will shoot straight with you on what I hear and why I like something or may prefer something else and why certain listeners may prefer a particular set of headphones or speakers, and some listeners might prefer something different.  

That said, I will do my best to compare these two headphones so you can get a better idea of the differences in sound and decide if one is a better fit for your personal preferences. 

One of the first things I notice when comparing the HD 660s2 with the HD 650 is the soundstage performance.  

The 660s2 depending on the song, generally has a subtle improvement and a more nuanced sound stage with slightly better separation, imaging and layering of instruments, especially when listening on my Chord Hugo TT2.  When putting both headphones on tube amps, the stages of both headphones sound more three-dimensional.

Soundstage

The soundstage and the overall sound of the HD 650 and 660s2 sound intimate, and which headphone sounds more intimate depends on the track and how it was produced. Listening to The Man in the Long Black Coat by Bob Dylan, the 660s2 presents a bit more warmth and intimacy for Dylan's vocals, whereas the 650 pushes his vocals a bit further back on the stage.   So it’s not a one-size-fits-all for vocal intimacy and will come down to the song's production.  For instance, a song like Wandering by Yosi Horikawa sounds more intimate on the HD 650 vs. the 660s2.

Midrange

The midrange of the 660s2 still sounds fantastic, and the headphones do so well in this regard if you are a midrange lover like me.  However, there are some differences between the HD 650 and 660s2.  The 650 presents the midrange as a bit more forward sounding with a bit more energy in the upper midrange so that guitar strings and some vocals have a bit more definition and edge.  

Whereas the 660s2 doesn’t sound quite as forward in the upper mid-range displaying strings with a bit more body and some vocals that fall into this range. If you felt the HD 650 had too much emphasis in the upper midrange, you may prefer the 660s2; however, both are highly enjoyable in the mid-range, and this will come down to your personal preferences. 

Also, I should mention that I listened to the re-badged HD 650 headphones from a company we shall not name, and it isn’t the exact same sound quality or build as the actual HD 650.  Listening to the original HD 650 headphones compared to the re-badged headphones, the re-badged headphones are the darkest and most laid-back sounding of all the HD Six Hundred series headphones, with the most intimate and collapsed three-blob soundstage.  

So, keep this in mind when you see impressions or comparisons online of the 660s2 vs. the HD 650 and double check to see if the reviewer is using the actual HD 650 or the rebadged version, as there are some subtle differences in those two sound signatures.  I have all the headphones on hand, so I was able to listen to them and compare. 

Bass

The next thing I notice when comparing the two headphones is that the bass of the 660s2 sounds a bit more detailed and nuanced but doesn’t necessarily present more bass quantity or sub-bass rumble than the HD 650.  Listening to Wandering from Yosi Horikawa, and both the 660s2 and the HD 650 are by no means bass-emphasized headphones; however, the HD 650 does present a bit more bass and bass impact to my ears.

So, although the bass of the 660s2 was improved over the original 660s set of headphones, you shouldn’t expect any of the HD Six Hundred series headphones to be bass-focused.  The magic of the 660s2 and HD 650 still resides in their unfatiguing and balanced sound signature with exceptional mid-range emphasis and overall spectacular vocal and instrument timbre first and foremost.

Treble

Treble-wise, both headphones are excellent if you value non-fatiguing and slightly pulled-back treble.  They are close in presentation, with The Sennheiser HD 660s2 having a slightly elevated treble response compared to the HD 650. The HD 660s2 also has marginally better imaging than the HD 650, which may enhance some listeners' perception of treble presence.

That being said, the difference in treble presence between the two headphones is relatively subtle. Other factors, such as your source equipment, amplification, and personal preference, can also affect how the headphones sound to an individual listener. Ultimately, listening to both headphones is the best way to determine which headphone has the sound profile and tuning that better suits your preferences.

Neither headphones have a bright or overly brilliant treble region.  I would still categorize both headphones as reasonably dark in the treble, especially when compared to the HD 600 headphones, which, if treble energy, sparkle and neutrality are characteristics you strive for in your headphones, you will likely still be best served by the HD 600 over these two headphones. 

For my preferences, I am still quite fond of the HD 650 and now the HD 660s2 sound signature in the treble region of the Sennheiser Six Hundred series headphones, especially when pairing them with more analytical DACs I own, such as my Chord Hugo 2 and Chord Hugo TT2 on filter two, directly from both components headphone amps or feeding the Woo Audio WA2or McIntosh MHA200 headphone tube amplifiers.

Regarding detail reproduction, this is very close between the HD 660s2 and HD 650, with both headphones presenting a bit more detail and treble energy than the cheaper re-badged version of the HD 650. 

Timbre

Timbre-wise, both headphones sound exceptional, and it will come down to your preferences as the HD 650 presents a bit more forwardness and edge-to-string instrument timbre and some vocals when they fall in the upper midrange. The 660s2 offers a bit more girth and body to those instruments and those vocals in the mid-range.  

Overall, the 660s2 does sound like the warmer of the two headphones for this reason; However, it doesn’t necessarily sound like it has more bass or sub-bass, it does sound a bit more detailed and nuanced in the bass region, and the additional warmth in the mid-range is delightful when paired with some tube amps and more analytical-sounding digital-analog converters. 

Speaking of tube amps, as I have said in other articles, there is just something so special about the HD 650 and the HD 660s2 when paired with tube amplifiers. Their soundstage dimensionality improves; there is a lift in that veil, so many audiophiles talk about allowing the headphones to scale in performance.

The mid-range sounds more open with a glorious and intoxicating sound quality, and the tube amplifiers I listened to helped some in the lower-end bass frequencies.  The six hundred series of headphones from Sennheiser truly scale with better quality amplification and the type of amplification you give them.  I am very fond of the sound of OTL headphone tube amplifiers with these headphones. 

Final Thoughts

In conclusion, I am a big fan of both headphones and need to add them permanently to my collection of headphones and will be keeping an eye out for upcoming sales as Sennheiser does run great deals throughout the year, so keep an eye on our website so we can pass on those savings to you!  

If you are interested in trading up your old headphones, amplifiers, or other gently used audio gear for a new set of headphones or speakers, check outour trade-up program, and don’t forget we will price match, other authorized dealers!  

We have some cool videos coming this year with new product announcements, so don’t forget to subscribe to our channel to stay in the know!  

From $150 earphones to multi-million-dollar home audio, cinema, and automation systems, TSAV is a HiFi enthusiast's paradise for building the system of your dreams.