Early impressions/notes... it's about on par from my memory of the uDAC-2. Decent sound, lots of gain (seems to do well without distorting at least), nice solid feel to the unit. Gain might be too high with iems, but I haven't tested those yet. With a Munitio Pro40 which is quite sensitive, there's barely much room for me to play on the pot
Compared with the HDP, well, the HDP is clearly a league above. Most noticeable is the uDAC-3 bass is looser. Almost feels like a bass boost but I don't think it is. It's a bit much with cans that are already flabby, but those lean in the bottom end don't mind it.
Coax output works just fine. I've been feeding that into my HDP without issue.
It is not immune to interference/noise from other usb devices. I tried playing music while transferring a movie on an external hard drive and music was constantly interrupted by clicks and pops.
With a modded T50rp (actually a transplanted driver) I can get to 3 o'clock on the pot before clipping. Volume is at the top end of comfortable listening for me at that point. I prefer to have it closer to 12 o'clock. With more sensitive cans I'm more around 10.