I got the 660S2 6 days ago and compare it to my 660S
(Sry for mistakes, English is not my main language).
Overall… I have to say (different to all the other reviews) they sounds not better than the older 660S.
(Other 6xx generations I never heared)
Some facts:
I use the new Fiio K7 and every time the 4,4mm balanced input
The 660S2 is a little more comfortable. (Less pressure - my head is a Motorcycle “L” size Head 
)
-The 660S2 really have some better deep sounds and bass
-Middle are same compared to the 660S
-Less stage than 660S
-And high sounds….
Well I think, the 660S are a little bit faded in that area.
The 660S2 not, but! the high sounds are more “stressful” in my opinion.
It is more analytically on the 660S2.
I don’t like that for my usecase.
(Gaming and casual songs)
Overall….
Yes! The S2 is a little bit better. But just ->a little bit<- (is is really hard to hear any difference)
If I look at the price in EU (350 Euro vs 600 Euro…) the 660S2 are not worth it. 
And I think I will return it.