So, I've not tried the loop, maybe an option...
Using both XLR and RCA seems to half the signal (volume out of the amp), but still using either individually can't get signal on the duet.
Temporary/botched solution, I'm running the mixer to the amp aux in and monitoring with the duet on the the tape out of the amp. I can thus get signal from the tape out on my amp to input 1 on the duet using the inst setting on the duet's input 1. (Why the inst setting and not the line?)
This seems unnecessarily clunky but I'll run with what works. Any other suggestions warmly received!