It all depends on the room and the drummer, really. I recently played a power-chord heavy jam with a drummer buddy in his new rehearsal space and brought my DRRI along. The room's ceiling sloped up and away from his kit, creating an amphitheater effect. He is an almost Bonham-like hitter and in this room, his drums sounded immense. When he got rolling, I turned the amp up to ten and it didn't even make a dent. It didn't help that it was sitting on the floor. So much for that.
I've played with the same guy on the same kit in a small sound-deadened basement and put the volume on 4/5 and it was plenty loud.
I was in a country band, doing the Luther Perkins thing mostly, and I'd barely have it nudged above its lowest audibility--even at unmiked gigs!
On mine, sound doesn't actually come out of the thing until you get it up over 2, then it kinda leaps out if you're not careful!
3/4 is still pretty clean, but actual "loudness" peaks out around 5/6. Turn it up beyond that and you're turning up a nice compressed overdrive. Granted, I've had the stock tubes replaced and re-biased so it's not so cold-sounding as it was originally.
I've never played through a Twin, but from what I understand, it stays clean and just gets louder and louder.
Good luck!