In the meantime, I realised that I have no idea if I'd even like a Feed-Forward compressor vs FET 1176-style. I've never used FF before. So I plugged the FM9 into the front of the Princeton with the Studio FF comp selected and...
Yep. REALLY good. FF definitely retains transients as advertised. I'm going to play a bit more with this too, and compare with the 1176. FAS models are so useful for exploring this sort of thing, even if the goal is to use a real pedal.
Background to this is that whilst the FM9 is fantastic into the Princeton, I don't want to keep moving the FM9 from it's spot in another location and shoving it in front of the Princeton, when I've got a mini-pedalboard there. First world problems!
But I do like splitting my time between the tube amp and the FM9 + monitors at the moment. 2 different little rigs for different moods and reasons.