well it has to be powerful enough to run the cab, which i think is 8ohm (?). i know valves can sometimes be a bit "louder" than tranny amps so im guessing a 120W valve amp would ruin the cab.
preferably i need a clean and dirty channel, and all my fx will be from guitar > fx> amp head, i never use an fx loop but i suppose if its there then theres always the option in future.
I use the amp for practice, which is bass, drums, keys and the vox (vox go through the pa) and it will also be for live situations, in which the amp will more often than not be mic'd up