The Ampeg B25B has to have an 8 or 16 ohm cabinet, so that will be an issue if you only have a 4 ohm cabinet. Also, the B25B is not as loud and clean as you probably want. I've never played a Sunn 200S, but it may be a tad cleaner/louder. It also has a 4 ohm tap.
See if you can play a 50 Watt amp in person, because you may not be able to get enough clean power out of 50 Watts to match drums and guitar amps.
Everybody stand back. I'm an Electrical Engineer. I'll take care of this.....