I just got my UHA-4 today with the standard op amp. I emailed Nick and asked him if any of them had a lower noise floor and he said that it wouldn't matter. I'm assuming that's because the one you can get changed is the output stage and is at unity gain with maximum feedback already and most of the noise will come from the gain stage. I don't think it matters too much but there's probably more variation using different op amps in the output stage than the gain stage the NwAvGuy was talking about. Nick has a dScope just like NwAvGuy so he could post some measurements of the different op amps if he wanted to.
Subjectively I'm enjoying the UHA-4 a lot so far. It has a much lower noise floor than my XM6. On low gain there's no hiss with my Shure SE530s from the analog input feed by my D2+ and a
very slight hiss from USB input. Its lower gain and perfectly balanced digital volume control mean I don't have to use the software volume control on the PC. The XM6 locked the Windows volume control and trying to adjust the volume precisely in the YouTube player is AFIK impossible and its a pain in the ass in Pandora. The UHA-4 doesn't lock it in case anyone is interested.
I'd like the crossfeed to be a tad stronger or adjustable like the XM6's but the UHA-4's seems like its good enough to keep me from getting a headache. The XM6 also has other nice features I used a lot like separate bass and treble boosts and an adjustable sleep timer along with other features I didn't use too much like an impedance switch, a volt meter to tell you the exact state of the batteries' charge, and even a flashlight mode. The XM6 also has more power but I didn't use 'phones that needed that power with the XM6 very often.
I haven't done any A/B comparisons yet but it sounds clean, extended, and detailed with both my SE530s and Beyer DT1350s. Low gain is also enough for the DT1350s from both my D2+ and the internal DAC.
Overall the UHA-4 is looking better for me. I was hoping it would outdo the XM6 in noise floor and pocketability and it looks like the UHA-4 will replace the XM6. I'll probably miss the XM6's extra features but the low noise floor and balanced volume control are more important than those other features.
You're already the Wood Maiden right? The beautiful Elven lass who lives in the heart of an ancient and mystical forest where headphones do in fact grow on trees and who brings down the wrath of Nature on any who transgress by harvesting the natural bounty. Many wish to ask for A/B comparisons among your extensive collection but only those of firm resolve dare brave the mortal dangers of the forest, only those possessing a sense of humility, compassion, and charity pass the required tests of character, and only those of the keenest wit will puzzle their way through the natural labyrinth to find your sacred glade in order to ask.
That's how I've always though of you anyway...