I'm running stock in mine, 8610. I had originally intended to use it with my phone, so I wanted to have something that balanced sound quality with immunity from (or, at least, less than utter permeability to) cellular interference. I haven't heard any of the other opamps, but as I mentioned, I'm very pleased with the performance as it stands-- and I do still get some occasional interference from my tablet or one of my phones, so I'll probably be sticking with the 8610 for a while.
You know, earlier in the thread there were several people who said that the DAC in the 6S was actually much better from its non-USB inputs-- and considering that the DX50 outputs coax and the 6S accepts it, I'm planning on giving that a run at some point. (Truth be told, I had intended to try that out originally, but forgot to take the cable with me on my trip, so I just went LO > LI instead). I'm interested to see if it sounds as good that way as it does in the current configuration.