The combo's control layout is as follows:
Rythm Pre - Lead Pre - Bass - Mid - Treble - Reverb - Rythm Post - Lead Post - Resonance - Presance
So, it doesn't have a master volume that controlls both channels at the same time. The Pre's pretty much control the gain, and the Post's pretty much control the output volume. I was used to having that, but I really don't miss it at all.
If you do decide on the 5150 combo, I would highly suggest you install a set of casters on it. Either that or get an amp dolly/cart.