after 2 day non stop listening , i can make some first impressions;
After owning almost all TOTL hp and almost all mid fi HP . IMHO this HD660S2 sound is the most close to lifelike sound presentation. It is a totally natural sound for my ears, no need to EQ. I sometimes go to Jazz Clubs and Opera life concerts and imho the HD660S2 is the most close presentation to them. Also I am a guitar player and I know how the guitar sounds in my hand when I am playing, the HD660S2 gives the same tonality and naturalness without coloring the sound. The HD660S2 has the Sennheiser house sound signature with the best tonality from their headphones.
It is a much more speedy driver than the older HD6 series. I think the transient speed almost reaches HD800 levels which gives a nice flow to the sound. The whole freq spectrum is super clear but it does not have spikes for my ear . Honestly this is the first pair of HP which I used 6+hour sitting in a row without getting ear fatigue. So tonality is spot on.
The low freq and dynamics are MUCH better than the old Hd600 and HD650 . it is almost focal clear lvl or dynamics and bass but with much better tonality.
Technical vise I hear all details but not as spread out like the hd800 and not as micro details but for listening music and NOT analyzing the HD660S2 gives much better listening experience .
Vs Focal Clear MG : the Mg got the lead on technicalities (but not much) and trades some tonality to dynamics, therefore the MG got more bite but more artificial sound it got this aggressive metallic sound in the upper freqs . The HD660S2 found the sweet spot in technical vise and tonality the HD660S2 gives much more lifelike sound , ( i mean what you hear in live concerts with your own ear ) honestly it is more lifelike which is i think the most important for me.
Meze 109 Pro: Honestly the meze is a super colored sound with aggressive treble and lacking technicalities and booming bass. For my ears the Hd660S2 got better speed, more flowing sound, more transparent AND super natural sound vs the ultra colored sound of the meze. The meze got a bigger soundstage, however the HD660S2 gives better depth z axis. The Meze got boosted bass and a really spicy treble (which is good for half -1 hour listening but after that I realized that mezes aren't for me due massive coloration).
hd600: Imho the HD660S2 improves everything for me TONALITY TOO. The hd660S2 has better speed, more details, more slam, more transparency , better soundstage, better depth...
HD650: same for sound.
(maybe the revieves are deaf or not listen lifelike music i do not know, if they say that the HD660S2 is not a step up from the existing hd6 lineup...))
Hifiman Arya Stealth: Well you can kill me for this. But for me the Hifimans got this plastic timbre which kills the naturalness of the sound. The HD660S2 has a better listening experience, better dynamics , and much lifelike sound. The Arya got a much bigger soundstage, (almost Hd800) the Arya got better micro details, more extended in both ways but it sounds like a plastic bag vs the Sennheiser house sound. Also the dynamics is non-existent on the Hifimans. ( the transient speed is better on the Arya obviously but this not gives dynamics to the sound)
I used Sennheiser OWN amp (HDV820) for most of the listening with my HD660S2. So maybe that's why I got these good impressions. I believe the Sennheiser guys tested the hp in this amp ( I know it has high output impedance which can alter the sound but they engineered it for the HD800S to give some slam and 3 d effects to the hd800 series ) . I also listened to my OTL dual mono tube amp which was really good paired with via chord dac (mojo2) (the OTL tube amps got extreme high output impedance which can alter sound). I am not an ASR guy. I just know as a musician how the live music sounds.
The negatives: cheap paper box, it has the same build q as a NEW hd600. My older hd650 which was made in Ireland got a premium feel and plastic in it, this new plastic in the Hd6.. line up gives this cheap feeling . (it not alters the sound but feels cheap in the hand )
Also when i Opened the box the cable which was installed to the hp ,6,3mm, was badly bent near the connectors . Obviously the hp can be damaged without a proper box in transportation. it is a negative. Also when I opened the box it was not a good experience . I remember when I opened my first old HD650 a long time ago and it gave me that satisfying unbox therapy feeling. Now opening the HD660S2 box for the first time gives a disappointment after I owned a lot headphones.
I love the flattened edges as well. 
So for now you use the Clear MG for hiphop/pop, the 650 for vocal and the 800 for classical ? May I ask if the 660s2 becomes a redundant hp? Or will you still compact your collection because of the 660s2’s sounding flexibility ? I have heard the MG. Its mids is simpler, stuffed and lacks the nuance of the Senn and Audeze hps
as i mentioned i sold almost all my HP. My top can was always the OG hd800. I like it the most. Also Hd650 was for otl warm chillin funn. but the HD660S2 better in every aspect. I wait Sennheiser to drop his new TOTL can. maybe it is happening 
sorry i never heard the NDH 30 . But according to this forum it is a good headphone and got a professional tuning for studios. Which means more analytical sound with elevated treble and not so warm tuning.
