I got an SU-8 on the way and my THX 789 is set to ship in May. My V1's are already here though.
According to one of the THX engineers, the balanced inputs are connected directly to the SE circuitry (lowering voltage at the same time?) so there's technically no SQ or signal difference between them AFAIK. I'm not sure it's the same with the 4-pin headphone output though. I haven't looked into that aspect specifically but if you look at most (if not all?) balanced+SE amplifiers, the balanced headphone output is noticeably louder.
The balanced 3-pin outputs on the SU-8 are however, slightly better (not by much) than the SE output (according to Amir on ASR:
https://www.audiosciencereview.com/forum/index.php?threads/smsl-su-8-version-2-balanced-dac.5433/). I would simply go with XLR (balanced) to avoid ground loops and/or noise issues, any audio difference that small is likely a perceived improvement rather than actual difference. That's entirely up to you though
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When it comes to volume and knobs... I run my V1's almost at the same volume level as my old AD1000X, which are 40 ohms. Now... if that's because of the FR differences between my headphones... or the fact that the Verum's are planars, I'm not sure.