Anyways, just wondering.
I know you want an effects loop too, but for a more high gain sound you might want to just get an OD pedal or maybe a Metal Zone, as those sorts of stomp boxes love all tube amps. The tubes sorta warm the sound up nicely.
Here's what I was thinking.
Guitar -> Preamp -> Effects Loop ->T.C. G-Sharp -> Preamp -> Univalve Lowgain Channel
The Uni is real nice in that it lets me put just about any tube combo I want in it. I thought of using a real low gain combo of tubes on the Input and Driver stages then the 6L6 in the final. This way I'd build up all the gain sound in the preamp and effectively use the Uni as a power amp. But yea, you hit on my concern exactly. The input impedance and level needs of the Uni versus the output of the preamp.
You still might try the MetalZone though; it has some pretty active mid level and frequency controls that allow a real scooped sound if you need it. I don't use mine much now, but for awhile there I considered it my 'Boogie in a Box' and it went last in the chain, right before the amp and after the rest of my pedalboard. With the MZ (or other high gain type distortion device) you effectively wind up with two different sounding amps, more or less.