There are roadies out there, some who hardly play themselves, but they have the knack. Matching an amp to the room is an art. All I know is that the portion of your amp that you use is vital. I have smaller amps that sound better than the large amp at home because they're at 80%, the large amp is barely moving the speakers. In a large room, the big amp takes over. Speaker movement / air volume changes everything.
Whenever you set your own gear up at a gig, you are making small sub-conscious compensations as you play the sound check. When playing normal with the band, an experienced sound guy can monitor and adjust the mix as you play, much better than the BP can.