The open-back Sennheiser HD 660S happens to be one of the few headphones at Moon Audio that had escaped my ears until recently. I know that it is revered for its neutrality and transparency, but I had never listened—that is, until I learned there was a Sennheiser HD 660S2 on the way. From that point on, I spent a lot of time with the 2017 original so that I would be well-positioned to do a comparison review. Known for its dynamic and forward sound, the HD 660S doesn't have the most robust bass response. Articulate bass? Yes. Thumping, deeply extended bass? No. This is something that Sennheiser chose to address with the latest version of this vernerable hi-fi headphone.

There have been many iterations in this line of headphones: the HD 580, HD 580 Jubilee, HD 600, HD 650, HD 660, and now HD 660 S2. It is one of the longest and most successful lines of headphones ever made. A fun side note: The first headphone cable that Moon Audio made (many moons ago!) was for the Sennheiser HD580/HD600. Needless to say, we have a deep fondness for Sennheiser headphones.

1) Sennehiser has significantly increased the compliance of the surround, dropping the resonance frequency from 110 Hz to 70 Hz.

>>What this means: Bass drops off below the resonance frequency in open-back headphones, so a lower resonance frequency increases the bass.

2) Optimized airflow around the transducer.

>>What this means: There is less air leakage between the front and back of the transducer. Air leakage introduces an acoustic short circuit, decreasing acoustic pressure for low frequencies.

3) The coil in the 660S2 has been revised to match the HD600 and HD 650.

>>What this means: The HD 660S2 has an impedance of 300 ohms (vs. 150 ohms on the HD 660s). This makes for better electrical damping, and the lower weight of the coil improves detail retrieval.

The result of these changes is that sound pressure at the lowest octaves has doubled. With more extension at the low end, more space is created for the sound. This has an effect throughout the frequency range; highs feel more brilliant, the soundstage seems deeper, and the music sounds more impactful. But there's still plenty of precision in the low end; the bass never feels bloated, nor does it bleed into the midrange.

The treble is smooth and easy on the ears. It's not revealing to the level of many top-of-the-line headphones, but it does a pretty nice job with detail and separation. From the classical guitars of Rodrigo y Gabriela to Scott Avett's banjo, strings have nice texture and musicality. On certain tracks with very complex instrumentation and intense dynamics, the HD 660S doesn't offer the level of resolution desired for intense critical listening. But this is a $600 headphone, not a $3,000 headphone. It still sounds pretty darn good.

The midrange is something of a show-off, with vocals in the spotlight on every track. From Susan Tedeschi's strong and sultry voice to Tracy Chapman's expressive contralto to Seth Avett's smooth and wistful vocals, I truly feel like these singers are close enough for me to hear every drop of spittle and every bit of texture in their vocal vibrations. There's warmth here, too, which adds a level of expressiveness and emotion.

OK, let's talk about the bass, which in my opinion is a big area of improvement vs. the original HD 660S. The HD 660S has been critiques for sounding too thin, with a lack of heft in the low end. Bass on the 660 S2 sounds more impactul, and more deeply extended. In But there's still plenty of precision in the low end; the bass never feels bloated, nor does it bleed into the midrange.

The improved bass was particularly enjoyable on Tedeschi Trucks' "Let Me Get By," a bluesy track with a mellow, smoky sound. I really felt the sub bass here. The upright bass and cello on The Avett Brothers' "The Once and Future Carpenter" gave me the shivers in a very good way. The former had a deep, pleasing pluck, while the latter had a gorgeous richness.

That being said, the HD 660S2 is not a bassy headphone. It's probably not going to be your choice for EDM, rap, or other genres that rely on thunderous bass. That's not what the HD 660S2 is about. But in my opinion, it offers a much better bass experience than the original.

A cable upgrade is a great way to add more clarity to your headphones, but different cables can also "correct" or enhance certain aspects of your headphones. With the HD 660S, I used a Black Dragon cable to add warmth and weight to the low end, smooth out the top end, and give the midrange a more musical sound. However, I found the Silver Dragon to be a more suitable match for the warmer signature of the HD 660S2. The Silver Dragon is our flagship cable, and it's great for enhancing detail and clarity while tightening up the low end and adding sparkle to the top end. It's a great match for warmer headphones and for those who really want to maximize detail retrieval.

Why Dragon Cables?

Generally, stock audio cables are manufactured with subpar materials, metals with impurities, poor geometries, and an overabundance of layers to make them look and feel like a fire hose. Inconsistency and lack of quality control in stock cables can lead to poor sound quality and a veiled sound vs what the musician intended for you to hear and feel from the music. Dragon cables are handcrafted with the highest standards and made to order according to your specific needs. At Moon Audio, we create a custom cable for you using the highest quality UP-OCC silver or copper conductors that can be manufactured. UP-OCC metals are void of impurities and are optimized for signal transfer and sound quality. We have one of the largest collections of audio and headphone connection options available online and we create limitless audio cable options depending on your specific gear and needs.

Silver Dragon Sound Signature: The Silver Dragon is the original cable. Silver strands clarify instrument separation, increase the soundstage, and find previously lost high and mid-frequency sounds. Transients appear more cohesive and the bass tighter for a more controlled sound. The detail and clarity of the Silver Dragon make it a perfect match for classical music and other genres with many nuanced instruments.

Black Dragon Sound Signature: The Black Dragon is warm and smooth with a musical presentation. Copper strands enhance the body, shape, and immediacy of the music. The detail and expansiveness of the Black Dragon make it a flexible fit where a broad range of musical genres are played. It can also improve bass frequencies on bass-light headphones. The smooth, musical quality makes it a perfect fit for headphones that tend to sound a bit edgy or bright. Despite its warmth, the Black Dragon does not have a laid-back, lush sound. It is much closer to neutral, making it a very natural-sounding cable. For more information on finding your signature sound, visit our Sound Signature Guide.

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