SENNHEISER HD660 S2 HEAPHONE REVIEW

The Sennheiser HD660 S2 Headphones cost just under £500 and in this review for HiFI PiG, Oscar Stewart puts them through their paces.

Sennheiser is one of the oldest names in the audio industry, making everything from microphones to high-end headphones such as the HE-1 system. The 600 series of headphones have always had a strong following with the HD650 and HD600 being used by audiophiles and studios across the world. The HD660 S2 follows on from the HD660s’ success and promises to address their shortcomings, they also strive to be one of the best headphones for under £500 – so how good are they?  

BUILD QUALITY, FEATURES AND COMFORT OF THE SENNHEISER HD660 S2

The HD660 S2 has similar build quality to the rest of the HD600 series with simple yet effective design choices and materials. The headband is plastic and reinforced with metal where needed, the arms and cups are plastic and the open-backed grilles are metal. There is subtle Sennheiser branding throughout and this time it’s printed in a rose-gold-like colour, the cable is detachable and uses the same connectors are the rest of the series. The 2 included cables are well-made and rubbery, they have good strain relief and I’ve personally seen these cables last a long time. The earpads are velour and, like most of the parts on the headphones, are easily replaceable when they eventually wear out. The overall build quality is excellent and knowing that you can replace parts is always reassuring, if they’re anything like the HD600 and HD650 then these will happily last you many, many years of use. 

Feature-wise you get 2 cables with them, one that’s terminated in a 6.3mm single-ended cable and another that comes with a 4.4mm balanced connector. You get a 6.3mm to 3.5mm adapter should the device you are using only have a 3.5mm output, and there is a small carry pouch included to keep them dust-free when not in use. These are open-backed headphones and do not isolate from outside noise, the impedance is up from 150Ohms to 300Ohms for the S2, indicating these do use a slightly different driver to their predecessor. 

A quick note on the impedance of these headphones, they are rated at 300Ohms and 104dB @1Vrms which does mean they are going to sound their best out of a dedicated headphone amp. It doesn’t need to be an overly powerful amp, but a desktop source will get the best out of these. Also, that magical 300Ohm impedance means these will work well with OTL valve amps, something the HD650 and HD600 both work exceptionally well with. 

Comfort-wise, the material and shape of the earpads make the HD660 S2 a very comfortable headphone in use, they clamp to your head fairly firmly straight out of the box but usually don’t take too long to loosen up a little in use. The headband is well padded and the headphones are designed to be quite lightweight. The included cables are nice and flexible and all of this adds up to a headphone that is easy to wear for an evening of listening to your favourite tracks. 

SOUND QUALITY

It’s been a while since I heard the original HD660s so I’ll try not to compare the two much, but I do recall them being a little brighter sounding and lacking a little sub-bass extension. The HD660 S2 has a more even lower treble region, and better sub-bass extension, making them very easy to listen to headphones. Most of my main testing was done with the SMSL D6s DAC into the iFi Audio iCAN PHANTOM > 4.4mm output into the HD660 S2, but I also tried them through the Miltra OTL-8XT valve amp. 

Eat Static – Uforic Undulance has a deep pulsating bass line during the intro, something you feel more than you hear, and the HD660 S2 manages to pull this off brilliantly, you can feel the pressure changes from these bass tones at normal listening levels. The swirling synths are easy to follow and there is good soundstaging, allowing you to locate each sound with ease. These are not the most punchy and dynamic headphones for electronic music like this, but they are enjoyable and keep control of the different elements of the track. 

I always seem to manage to include a Lights song in my reviews, they seem to pop up at the right time and I’ve heard her vocals through so many systems, as well as seeing her live, that I feel I know how she should sound. And Counting… is quite a slow, brooding song, and through the HD660 S2 you may as well be in the studio during the vocal tracking in my opinion – you can hear every minute detail through these. The midrange is excellent and manages to sound very natural in its tonality, there is a very good level of separation allowing the midrange to always cut through the mix with authority. 

I would say these are mildly mid-forward headphones – they sound fantastic with both male and female vocals and are very smooth when listening to acoustic, without missing out on the detail. They do, however, hold up well when it comes to the rest of the sound, out of a solid-state amp you end up with a slightly dry presentation in the bass region which does make kick drums sound a little flat, but bass guitar lines come across with good articulation and body. Snares are snappy, electric guitars have good crunch when required, and the treble is crisp and well-extended. 

The Appleseed Cast – Blind Man’s Arrow. For this track I put the iCAN PHANTOM into Tube mode, this imparts a hint of valve magic into the sound and made this song come alive. There was excellent separation between the different guitar lines, kicks had a slightly fuller impact, and the treble was beautifully presented. This allows the HD660 S2 to deliver every tap of the cymbals without bringing them forward into the mix, they allow you to hear the finer details in the treble without drawing your focus to them. 

Moving on to the Miltra OTL-8XT I found the sound to be a little fuller and warmer, but without taking away the excellent detail retrieval of the HD660 S2. CHVRCHES – Killer sounded full and yet well controlled, the bass doesn’t sound bloated even though it has slightly more presence. The vocals, once again, stand out beautifully with their accuracy and tonality, there’s a realism to them that is incredibly enjoyable. The treble maintains its crispness and still extends with ease, rounding off an excellent sound signature when used with valves.

Dashboard Confessional – The Best Deceptions (MTV Unplugged), the warmth and body from the acoustic guitar are beautifully rendered by the HD660 S2, yet his vocals cut through with the raw emotion he’s known for. Every fingertip sliding up the fretboard, every strum of the strings is raw yet real, there’s no glossing over of details but, these are also not a microscope wanting you to pick out flaws in your favourite tracks. The audience’s participation encircles the main sound with great width and depth, these make it sound like you are sat in the front row in terms of their staging. 

The HD660 S2 is a very complete and easy to listen to headphone, they are great for many genres but perhaps lack a tiny bit of dynamics when it comes to the heaviest of metal and hardcore electronic music.

QUIBBLES

The clamping force is a little tight out of the box.

Whilst they do well with most genres, they are not the punchiest headphones for more dynamic genres. 

CONCLUSION

The HD660 S2 does fix a lot of the issues people had with the original – there is better sub-bass presence and the treble is a little more linear. They are still a Sennheiser though, and they do still have a slightly dark sound signature. I cannot fault these when it comes to realism in the midrange and their beautiful way of rendering acoustic music, but when it comes to music that’s a bit punchier, they do sound ever so slightly flat. 

These are real easy-listening headphones, they provide a very natural and easy-to-like sound that excels when listening to acoustic, softer rock, jazz etc… They do not have a studio flat response; they have a slightly mid-forward sound that is backed up with a full and solid bass response and well extended yet fatigue-free treble. The sound staging is great, they go for a slightly more intimate presentation that sits you right in front of the stage with excellent width.

I’m impressed that Sennheiser nailed the sound description on their website – “Enjoy your music on a completely new level. An intimate, relaxed sound signature combines with outstanding precision and exceptional comfort — and now, extended sub-bass — for a deeply moving experience.”

If you do fancy these, I also recommend testing out a nice OTL valve amp with them, personally, I feel that valves bring these headphones to life a little.

AT A GLANCE

Build Quality And Features:

Built to last, most parts are easily replaceable

2 cables, 1 x 6.3mm single-ended, 1 x 4.4mm balanced

Open back with comfy velour earpads

300Ohm makes them a great match with valve amps

Sound Quality:

Smooth and fatigue-free

Natural and expressive

Open and well extended on both ends 

Value For Money:

For under £500 you get a very likeable headphone that’s natural sounding and comfortable, which makes them very good value in today’s market. 

We Loved: 

The natural and smooth sound signature

Great comfort for long listening sessions

We Didn’t Love So Much:

Slightly high clamping force out of the box

Sound a little flat out of some sources with punchy music

Elevator Pitch Review: The Sennheiser HD660 S2 are superb sub-£500 headphones! They give you a natural and real sound with great extension from top to bottom. They are such easy to like headphones that rarely put a foot wrong, and for that reason, I can easily recommend these for their price. 

Price: £499, 599 euros

Oscar Stewart

SUPPLIED BY SENNHEISER

SUPPLIED SPECIFICATION

• Transducer principle – dynamic, open
• Transducer size – 38 mm
• Frequency response – 8 – 41,500 Hz
• Impedance – 300 Ω
• Sound pressure level (SPL) – 104 dB (1 kHz, 1 Vrms)
• Total harmonic distortion (THD) – <0.04% (1 kHz, 100 dB)
• Ear coupling – Over-Ear
• Weight – 260g
• Adapter – 6.3 mm (1⁄4inch) to 3.5 mm
• Connector – 6.3 mm (1⁄4inch) jack plug, 4.4 mm balanced plug
• Cable length – 1.8m