Well the 660S2 is an interesting switch from the HD650. More technical detail, soundstage/imagine is definitely greater, even if subtle in some tracks or more obvious in others.
Since when did Sennheiser bundle in a 4.4mm balanced cable too in the box?!! Both cables in the box were also 1.8m long which was a surprise as reviews slated the super long cable in typical Sennheiser fashion?
The "Sennheiser veil" is immediately obviously swapping between Ananda Nano and 660S2, though the high range peak harshness in some tracks is well under control on the 660S2 vs the Nano which is a typical HiFiMAN trait as I've now learned.
The cups and headband are more comfortable on the 660S2 (as are all HD6 series) for long session listening. It could still be that my Nano headband and pads have not broken in yet and softened so still waiting for that.
660S2 has much better sub bass than the HD650 too, impressively technical in detail and articulation. But pausing a track and flicking over to the Nano you are immediately enveloped by soundstage and imaging that Sennheiser can't do. So it's a toss up between that textured midrange but put up with the veil, or have the high range brightness of HifiMAN. Even the Susvara/Arya Organics still have the brightness so it's not like the £1200 models solve that. I did try Peace/APO to drop the 7800Hz and 12000Hz by about 2dB but felt that this changed the musical dynamics. Instead I've discovered that simply lowering the volume slightly seems to help tone down any overly bright tracks. So instead of listening really loud, just lower it a bit and enjoy the imaging/soundstage.
I think the 660S2 will be going back for a refund, just wanted to hear them directly back to back with the nano and see how they compared, plus give me an idea of what they are like vs the HD650 too. I would say if you have the HD600 or 650 then the only reason to get the 660S2 is if you must have the added sub bass as this is something the HD650 can't match even with the mass loader bass mod.
The Nano has an eerie ability to combine imaging, soundstage and sub bass in such a way that the experience in my head is on another level, just tone the volume down a bit on harsher tracks that fall into that 7800/1200 peak brightness and all's good. I never imagined that bass like this could be heard and felt in headphones lol. The HD6 series just feels too narrow for soundstage after listening to the Nano, had I not gone through this journey then the HD6 soundstage and imaging would have remained excellent because it is, but the Nano is just on another planet.
Interestingly through the Fiio K7 at the same volume dial on the amp, both Nano and 660S2 have the same (or very similar) loudness. That's 14 ohms vs 300oms spec difference between them, madness.
I do love the all matte finish on the 660S2 though, shame Senn do not give a box or case any more, just some cheap-ass cardboard box and some cheap-ass "pouch". At least they give a balanced cable now and don't give you a 10 mile long secondary cable.
I will say this, the Sennheiser plastic is superior to whatever cheap-ass feeling plastic HiFiMAN are using. I have no idea why they didn't use all metal considering they did just that with the Sundara... It boggles my mind as to why the Nano isn't all metal too.
Edit* Listen to this playlist on headphones, 3D audio at its finest.