Sennheiser HD 660S Headphone Hands-on Review: Top-class Sound with a Catch

For headphone enthusiasts, the Sennheiser HD 600 series won't need introduction. Famous for offering transparent, immersive sound, the HD 600 was followed by HD 650, then HD 6XX in a collaboration with Drop.

Finally, Sennheiser now brings the HD 660S as the German audio maker's latest offering of the series. Can it continue the winning streak they've been on since the release of the HD 600 in 1997 - more than 20 years ago? Let's dive in to find out.

Timeless Design

For those familiar with the Sennheiser HD 600, or HD 650, the HD 660S will feel right at home. Both the straightforward overhead headband design and oval ear cup design with the large mesh coverings are almost identical to its previous generations.

There are however some minor iterative changes, like the inclusion of a Sennheiser logo on the mesh - which might seem like a minor detail, but these markings are a fast way for me to determine the right and left cups. With the previous generations it was hard to tell the two apart, so the logo comes as a small added convenience.

There is also a change in its finish, with all signs of the glossy plastic gone, being replaced with a more pro-audio looking matte black. There are no other colors on the finish besides the black and the silver in the logo and model number. Personally, I much prefer the all-black, no-nonsense monochromatic color scheme, since it won't have a problem fitting in with any desktop set up.

Simple and Durable Construction

The Sennheiser HD 660S construction is as simple as it is durable. It makes perfect sense for the latest model to keep most of the design elements that its predecessors had, since it's proven to be so long-lasting (I have a friend who still have his 90's issued HD 600 in full working order). As the saying goes, "If it ain't broke, don't fix it".

Most of the HD 600S lightweight body is made of matte black plastic, with the exception of the adjustable stainless-steel headband. The width and thickness of the band gives me assurance that it will stay functional for at least a good 20 years.

Very Comfortable for Long Sessions - Even for Larger Heads

With my head circumference being 24", as a spectacle wearer, I do have trouble finding comfortable headphones from time to time, especially ones that I can wear without fatigue for long listening sessions. The HD 660S is hands down one of the most comfortable headphones that I have worn, with minimal pressure applied to the side of my head. The oval ear cups perfectly fit around my ears and they don't feel uncomfortable even if I'm wearing my glasses.

Despite the comfort of the HD 660S, there's interestingly nothing overly fancy in its padding department. Both the cushion on the headband as well as on the ear cups are made of regular foam rather than memory foam. I initially thought the ear cups might be covered by Alcantara, but they turned out to be velour pads. Conclusively, the HD 660S haven't used any special materials, just excellent design.

Amazing Sound - With a Catch

Honestly speaking, I held on to the Sennheiser HD 660S for a long time before I could form an opinion about it. Throughout my time reviewing it, I paired them with various DAC/Amps and DAPs that pass through the office, including a SPL Phonitor xe, Schiit Jotunheim 2 with True Multibit, as well as a FiiO M11 Plus LTD and M17 DAP. After pairing the legendary headphone with these sound sources - many of which costing more than $2,000 retail - the result I got each time was a neutral, laid-back sounding pair of cans. I was completely perplexed as to why they've been so well reputed among audiophiles for over 20 years.

That is until I received the new FiiO K9 Pro DAC/Amp. When using the 4.4 mm balanced output, with the gain set to high, I felt I heard the HD 660S for the first time. The dynamics were ultra-realistic. The instrument separation and sound staging were one of the best I've heard, and the overtone and texture were jaw droppingly good. Doubting my ears, I spent considerable time doing A/B testing between different sound sources, as well as comparing with the Audeze LCD 2C that I had on hand. The outcome was the same every time: the Sennheiser HD 660S paired with the FiiO K9 Pro is hands-down one of the best listening experience that I have ever had.

Amp and Headphone Match Made in Heaven?

But why? I put on my investigative hat and did an extensive dive into the possible reasons for this. If it's only about power, the SPL Phonitor xe is rated at 5W x2 at 300 ohms (the HD 660S is rated at 150 ohms) while the K9 Pro is at 1.1W. Here, the balanced cable could have been the reason, as the included 4.4mm cable with the HD 660S could only have accessed balanced sound from the K9 Pro 4.4mm output. Then, how about the M17, which matched the power of K9 Pro in its desktop mode? It turned out that at 300 ohms the M17 output dropped to 500mW, less than half of the K9 Pro.

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Conclusively, when the conditions are right, and when the Sennheiser HD 660S is matched with the FiiO K9 Pro using the included balanced cable, you get what seems like planets aligning, and end up with a beautiful explosion of sound. The catch with the HD 660S, in my experience is that though its resistance is half of that of its predecessors (both the HD 600 and 650 had a 300 ohm resistance), there needs to be power to fully open up its sound, and there needs to be a balanced connection.

Excels in Acoustic, Jazz and Classical Music

Just because the sound is amazing, doesn't mean it's the best sound for any kind of music - but it's pretty darn close. The best music the HD 660S is suited for are ones that make use of soundstage and instrument separation.

For example, acoustic music with vocals generally excels, well layered instruments and vocals laid on top. I took out an old Cowboy Bebop OST album from The Seatbelts/Yoko Kanno that I'm very familiar with and heard it in a completely different light. The jazz/blues funk tracks feature a range of instruments from trumpets, oboe, percussion, drum, electric guitar and more, and the HD 660S captured each texture with a pretty sparkle.

An extended listen of Johann Sebastian Bach's Orchestersuite Nr. 3 (BWV 1068)proved to be equally exquisite. Each string part is presented separately, harmonizing and resonating with one another. Even the sound of the cellists' breathing is presented very clearly as if it were part of the music piece, again demonstrating its excellent separation of instruments and, what's more, making it an incredibly immersive experience.

Final Verdict

The Sennheiser HD 600 series has been around for more than 20 years, and there's a good reason for that. The latest HD 660S has a very impressive sound - if you get the right headphone amplification. When you do get a good match - as we had with the new FiiO K9 Pro - its sound quality matches that of much more expensive set ups.

If you like natural sounding timbre, clear instrument separation and immersive soundstage - the Sennheiser HD 660S is the one to get. Considering its retail price is less than $500, this is one best-value-for-price that you'll find in the HiFi market. We're awarding it with the IBTimes Exceptional Value Badge

David is a tech enthusiast/writer who is often on the move, but since the lock-down is on a mission to explore gadgets to improve his 'home work'. This is a contribution to an ongoing IBTimes review series on gadgets for Home Productivity.