What actually controls the gain on a Marshall Vintage Modern? If I put the body and detail higher, will that simply result in more gain, or are they more of a tone shaping tool and you are supposed to use the guitar volume to control gain level. (I know that gain has to do with the amp's preamp, but what I mean is, will putting guitar volume on 10 and body and detail on 8 result in a more distorted sound than guitar volume on 10 and body and detail on 5, for example. I hope that makes sense.)

Yes, body and detail are gain knobs, but they're tone-shaping gain knobs if that makes any sense. Body controls low end and detail controls high end.
Quote by Trowzaa
I can't look at anything starting with BB and not thinking big busty.
Big Busty Queefing.
the vintage modern is kinda interesting.

it's actually get 2 input preamps.

they are voiced differently one is the body and the other is detailed.

then they go through a valve mixer and get mixed together before the eq stage.

that being said...you're not going to get that much gain out of that amp in the preamp.
Prs se Holcomb is the answer