RNB,
I don't consider myself a "golden ears" guy, but I have built a few amps and have a few sets of the OPA627 and AD86x0's to compare, plus OPA2134 and some others. I think this might be a good analogy...
When you hear a close miked cymbol crash, if you like to hear it buttery smooth, you will like opa627. If you like an edge to it, you will like AD86x0 better.
That's one of the easiest differences I have noted that I can articulate and I think this carries down into lower frequencies to some greater or lesser extent. I suspect that OPA627 has better bass impact. Not overall bass, but that slam that should happen when it should happen. I don't like boomy bass but I like my head shaken side to side when the music wants that to happen.
BTW, I use Senn 650's for most of my listening.
It's like asking someone if they like vanilla or chocolate better; I think it is a matter of taste.
It is interesting that, frequency response-wize, these chips are all flat from DC to the speed of light, at least as far as the human hearing range is concerned. Same with the other major specs like distortion. There is none on any of them, for all practical purposes.
There is one measurable difference, though, and that is power consumption. OPA627 consumes 7-8 ma per channel and AD86x0 consumes about 2.5-3ma. For that reason, I favor the AD86x0 in my portable Pimeta, which has 3 channels plus 3 buffers. It's a lot easier to feed the beast. I think I am going to keep OPA627 in my desktop amps and use AD86x0 for my portables and then I get a little change of pace, soundwize and I can optimize the battery consumption. I know you understand battery consumption because you have the Micro