From the manual:
"The OUTPUT control is joined by our SOLO feature that provides an additional master output that you can trigger remotely for a
footswitchable boost when you want to feature a part. These two Front Panel controls are brought on line with the activation of the Effects
Loop. When the Loop is not active, the individual Channel Master controls serve as fi nal output controls. When the Loop is set to HARD
BYPASS, all associated circuitry is removed (for you purists) from the signal path. Since the OUTPUT and SOLO controls are wired as
Loop Returns, at the end of the preamp and Loop signal path, they are defeated when the Loop is bypassed."
Hard bypass removes all the circuitry of the effects loop as well as the Output and Solo pots. This seems to include one preamp tube but I'm not so good at reading the tube charts.
I tend to leave the loop in; the Output knob is very useful in gig settings and the solo knob is great.