I did buy this second hand. No manual was present, and none on the Ashdown site (and none found on the internet). Ashdown did get back to me and suggested a solid state preamp (IMG Stage Line MPA-102). It's from Germany and I'm in the US. I tried an ART Tube MP and it was awful. Now using a Rodyweil R003 Microphone Preamp Booster (solid state) and it sounds much better.
BTW, I opened up the unit to find a JJ ECC81 in V1, but two identical unlabelled long-plate tubes in V2 and V3. I'm not sure if there was a swap made. I've got a set of all three JJ's (ECC81, ECC82 and ECC83) coming in and will see if that makes any improvement.
I will re-try the USB-C with a Thunderbolt 4 cable and will try the Windows Audio setting.