On the issue of master volume: For a few weeks, I'd been setting it to 100% and then adjusting volume in Canola, either low if using headphones or 100% when connected to the little stereo. I notice that the master volume keeps resetting to 85%. I haven't been able to determine what event might make that happen; I thought maybe insertion/removal of the audio plug might do it, but that doesn't seem to be the case. I've just decided to let it remain at 85%.
In my usage of Canola, the volume always remains where I have it set. I don't think my MP3s have Replay Gain volumes set; that might be related.
Regarding the audio-passthru mode: to me, it sounds like this is getting far afield of ASUI's original plan. I'd hate to see this great utility succumb to feature creep.