Yes, just a volume boost. I believe the only control you have for the loop is the level of the send. It isn't a mix knob, and it is truly a serial loop.
This is a great amp if you want a killer Marshall sound. It has the best clean channel of any EL34 amp out there, too. As with all Marshalls, it has some truly piercing highs, so most folks turn the presence and high eq knob down to a very low setting. (That's just a Marshall thing.) I prefer the Rectifier cabs to the Stiletto cabs since they give more bottom end and seem to tame the highs a bit.
This amp definitely sounds best without the effects loop engaged (no solo switch available) and used as a "master volume" head. Killer sounds to be had there. Still sounds great with the loop engaged, too. I haven't had mine very long, but I'm loving it more every time I play it. Great amps!!!!
-Brad
