If I put a delay pedal in the loop of my Seminar head, the volume drops when I engage the pedal, then jumps back up when I go into bypass - any tweaks I can do?
That's strange, I run three delay pedals in the loop of mine, a Ibanez EM5, Maxon AD900 and a BOSS DD-2 and don't have the same issue. I did have a noice issue in that if I plugged anything into the loop it would hum loudly, that turned out to be a ground wire that I didn't solder very well initially and eventually loosened up after the amp had seen some miles/gigs.
Shoot Bruce an email, he answered my question that same day and after 30 minutes of work on my part it was back to running as it should be.