Well, for anyone interested, I didn't manage to get the NAD M51, someone got there before me, and I've just dropped about $150 on a
Olasonic Nano-D1. New, in the box, even though it's a model from 2013. There's very little info to be found about it, but it's designed by a well known Sony audio guy, Yoshinori Yamamoto, and it went for $5-600 a few years ago. Very basic digital-in, analog-out. Haven't heard it yet, but I'll report back.