Just got my HD660, order to delivery time was 1.5 days
A few hours ago, I was still "married" to my HD650. No other headphone could ever please me more than the HD650. Most headphones are very problematic in the treble area (too sibilant, splashy, a constant zizz etc.), and miss the richness of the HD650 treble. On the HD650, the different cymbals of a drum kit do sound different, and expose the tonal differences in their decay. On top, a lot of headphones miss sufficient energy in the fundament area (chest, instrumental cabinets), either bass heavy (thick), or lots of mid (tonal rich), but most expression and push in the sort of music I like comes from the fundament area (voice, piano, guitar), so this is an important aspect for me. Headphones like the HD800/700, Philips Fidelio (close though), AKG 7xx, Beyer Dynamic 8xx/9xx all don't work for me for that reason.
So why an HD660 then? Yes, there is definitely some "muddiness" in the HD650 sound, the fundament region (chests, cabinets) does melt with the harmonics further up in the frequency regions and masks details. It does sound very rich, but in busy rock music it can start to sound "stuck", loosing the liveliness of the performance. Furthermore, the top end does sound a bit recessed (rich but not sharp), and the bottom end of the bass is rolling instead of punching. Not to the extend that it is disturbing, overall I think the trade-offs are still very good compared to other headphones. The HD600 sounds a bit more "sharp" and less "wooly", but I was always disturbed by its edgy treble, that makes different cymbals sound more alike (exaggeration makes my point clear, in case an HD600 admirer feels a bit upset
- the HD600 is a nice pair of headphones, but not very refined in that region in my opinion, it is treble in quantity and not quality, and it lacks in the fundament region in my opinion).
So I took the risk to order an HD660, knowing that when ordering it via the web I have a 2 weeks guarantee to send it back for full refund according to the protective laws in our continent. No need to do so! The HD660 is wonderful!
First of all, it has a somewhat different timbre. Less "warm", but certainly not analytical, and also "less hollow". In the beginning I thought it was a bit "thinner", but this is not the case. There is less melting of bass/fundament/mid-range. The bass is there, the fundament is there, the midrange is there, but more independent, and better harmonically structured. For instance, when listening to a Viola Concerto of Brahms (Zimmermann and Gernstein, label Myrios Classics) you can hear the different elements of the piano much better, what is the snare, what is the cabinet, what is the reverberation, the panel resonances, etc.
This brings me to the second point, the spatial projection is a bit bigger, not because it creates an artificial large space putting everything more distant, but you can better distinguish the direct sound from the reverberant sound of the players in the recording place. This adds to the quality of sizing instruments better, on the HD650 there was the tendency that instruments projected more to the same size (again, exaggeration makes my point clear, on the HD650 it wasn't bad, on the HD660 it is better). It also adds to a better projection of "silence", the decay and dying of tones is "more explicit" and "more positioned", as if there is a lower noise floor. Also side-sounds (fingers moving guitar snares creating scratchings sounds, musicians turning sheet papers of the scores) is explicitly there now, it draws your attention (but doesn't distract, it is not magnified, it sounds natural).
Bass is tighter, but also a bit less (in the 80-150Hz region). There is plenty and enough, the HD660 reveals the deeper bass better, not in magnitude, but because it is not masked. When listening to it a couple of hours, and going back to the HD650, the HD650 sounds "coloured" in that region, as if you listen to a speaker with cabinet resonances/reflections, where the HD660 sounds as if there are hardly any resonances. That might be confused for "thin", I perceive it as "more correct".
Then, the treble is a big plus of the HD660. It is "sharper", so there is a lot more fun with cymbals et.al. in the higher treble regions, but without the famous "treble becomes fatiguing" problem (it even passed the "Innocence Mission" test with the over-harmonized song 'When Mac was swimming' from the album 'Befriended'). It has the same type of richness that you also hear from high end speakers (with properly filtered diamond, ceramic or beryllium cones), where you suddenly hear drum kits consist of many different cymbals, and brass can sound very different under different angles and play styles. Female voices have more "sigh", more "ph", and "moving lips". I can put the HD660 much louder on pop/rock records than the HD650, that start to melt the fundament/mid, and the lower treble becomes somewhat pressing on the ears. Therefore the HD660 gives a more dynamic appeal. This is very clear on drum beats, more instant, and with more kettle sound. It is not a head-banger headphone or a punch-the-bass-on-the-eardrums experience (luckily not). But Radiohead, Rush, Michael Jackson, Fleet Foxes, Diana Krall, etc. etc. all sound perfectly fine, and can be played loud without tonal stress. The micro-contrast as heard on authentic instruments in for instance string quartets give a tremendous rich sound with a lot of micro-contrast.
Overall, the HD660 sounds less "romantic" than the HD650, and I think it sounds more honest, with a richer tonal structure over the full frequency range instead of only in the fundament/mid range. It brings a lot more excitement by exposing the subtle tonal expressions musicians add to their interpretations. A high music-is-fun factor. Cool, I'm happy if that wasn't already obvious.
Is it worth the money for an upgrade? My initial impression is it definitely is. It is not a completely different animal than the HD650, it still sounds as a HD6xx product, but it improves the weaker points of an HD650 significantly, and brings no new weak points in return. Only if you like the bold sound in the upper-bass/fundament of the HD650 you may miss that a bit. But it is not only worth the investment because of those improvements, but also because it brings the HD6xx sound to my mobile equipment. I usually listen on my computer via an iFi Micro iDSD (worth every penny for driving an HD650), but my (EU volume capped...) iPhone is able to drive the HD660 to sufficient levels with the same fun factor.
All my opinion and trade-offs of course.