If you have the correct ohm speakers connected the amp will be plenty loud enough for a rehearsal room. There are a couple of other things to consider, different speakers have different efficiency ratings, as bobbsy said, this means that they put out a different amount of dB for the watts put in. Also if you are running say a 12" speaker and a horn driver the dB will be a lot more than just a speaker as the horn driver has a much greater dB output per watt than just a speaker. The design of the actual speaker box also has a direct bearing on the efficiency of the speaker.
Now the proof, in our rehearsal set up we have 2 fold back wedges, each wedge has 2 x 10" and a piezo bullet horn. The amp is 200 watts a channel at 4 ohms but the wedges are 8 ohms each so we only get about 140 watts a channel. Due to the fact that the wedges are very efficient (they are part of a larger PA and have been on tour and coped very well and actually sound very good) we can very easily hear the vocal above the drums, guitars and bass. However one point is that we only use the fold back for vocal.
Another thing to consider is the room acoustic, a very live room tends to make everything sound loud, thy deadening the room so that the sound does not splash around, and finally turn down the amps and get the drummer to play softer, because at some point you will be at a gig and the manager will come up and say "you guys are too loud, turn it down", so you may as well find a good stage volume that works.
Oh and one last thing, the peavey has a limiter circuit that protects the speakers if you push the amp too hard, make sure this is switched on.
Alan
Alan