I have both. 660S sounds very nice with Harman target EQ. Getting bass EQ to my taste takes effort though, haven't gotten it perfect yet. It can't go as loud as 560S with big bass boost. At least I don't dare to push them further when I start to hear clicking on 660S. Different driver angle also makes them sound more like closed backs. Haven't had comfort issues with 560S ever, but 660S sometimes feels uncomfortable. I've had scratching sound on 560S, I suspect it's from hair getting inside cups and touching driver. Goes away after shaking the headphones. 660S has thicker foam protecting driver and no such issue so far. 660S feels like it has more premium build and will last longer.Another question, how do folk find these compared to HD660S?
560S is more physical. Probably because it can go louder and drivers are closer so you can feel vibration. I can play this on max vol, high gain on Atom AMP (older version, not plus), it's insane:
Preamp: -12 dB
Filter: ON PK Fc 60 Hz Gain 3 dB Q 1.3
Filter: ON LS Fc 130 Hz Gain 10 dB
Filter: ON PK Fc 150 Hz Gain -1.1 dB Q 0.8
Filter: ON PK Fc 1320 Hz Gain -2.8 dB Q 1.6
Filter: ON PK Fc 1850 Hz Gain 2.2 dB Q 1.3
Filter: ON PK Fc 3230 Hz Gain -3.1 dB Q 4
Filter: ON PK Fc 4450 Hz Gain -3.2 dB Q 5.713
Filter: ON PK Fc 5490 Hz Gain -3.5 dB Q 3.1685
Filter: ON PK Fc 7000 Hz Gain 2.5 dB Q 1.2
Filter: ON PK Fc 8400 Hz Gain -3 dB Q 4
Filter: ON PK Fc 11000 Hz Gain -3 dB Q 8