I got my HD660S, some thoughts on them, running with a Fiio K5 Pro and listening to music on YouTube like the peasant I am -
Out of the box they definitely sound a bit more veiled and damp than the Sundaras, I wouldn't say they are worse though. HD660S is definitely an accurate pair.
I don't have that annoying high pitch crackling and peaks I had with the Sundaras. I could listen to these for hours on end.
For the Sundaras, "S" sounds were terribly painful and loud, so are gunshots in game, as I said before.
The sound is more "in my head" than on the Sundaras for sure. I can still hear the location of the instruments. I guess that is what they mean by sound stage.
I equalized with Equalizer APO to boost bass (tune everything else down), I get a comfortable rumbling.
For me though, for the stuff I use this for, neither this nor the Sundaras were a pair I would lose my mind over so I guess this is kind of my price limit, for the applications I use headphones for.
Would the Sundaras not have those painful pitches and out of order crackling, I'd kept them. But for the particular items I received, the HD660S is a lot more consistent. I might have received a faulty pair of Sundaras.
I need to adjust the clamp force somehow though. Sundaras were very comfortable, the HD660S clamps a bit hard to me with glasses.
Experience for games, namely Overwatch -
The HD660S sounds exactly the same as the Sundaras, apart from that already mentioned sharp and loud high pitch sounds. I hear everything just as clear around me as with the Sundaras.
For some reason even with boosted bass my el cheapo Plantronics Blackwire has more present bass than the HD660S - could someone recommend some Equalizer Apo settings?