My HD660S arrived today and straight out the box they are the lightest, most comfortable and best sounding headphones I've tried. I'm not normally into headphones, preferring IEM's but considering no burn-in time, these sound better than the AT M50 I had briefly, the B&W P7W and I also tried the HD800 but found that not to my taste. I still think my SE846's sound better but then I suppose they should at double the price new. I actually think I will be keeping these.
Now the decision on the ZX300. I've decided to return as I realise just how much I need Tidal streaming compared to my own tracks on SD card. I can hook it up to a phone as I've shown in this thread but not ideal and much less portable/light/small compared to using my Z5C/DFR combination (photo below).
The lack of search facility was also a surprise to me not having used Walkman OS before. As I've said earlier, the DFR also sounds better than the ZX300 by SE and is not far off the Balanced connection either (accepting ZX300 can improve further with burn-in). Sony also got back to me and nothing definite on enabling MQA as DAC from Tidal which my DFR already does with Tidal PC.
The final thing that has made me realise its not for me, I don't think that 150Ohm is particularly high for headphones but the ZX300 can hardly drive the HD660S with authority and even at Max volume/High Gain its not too loud to listen to. It can cope but only just. I'd say your looking at 100/120 balanced with High Gain for many tracks. The Dragonfly Red by comparison easily drives the HD660S loud at 3/4 max on phone and when DFR connected to a PC playing Tidal MQA only 30/100 .
If Sony develop a player with the size/style/battery life of the ZX300 with Tidal/Spotify streaming support I'd be interested again, It does sound great over balanced with my SE846's