When I first joined my current band they rehearsed at ear splitting decibels. The drummer is great, an actual musician as opposed to a guy who just whacks the skins in time, but uses logs for sticks. To compete, the other guys would crank up their amps. No one was using ear protection.
As soon as I joined I refused to rehearse at those levels; I maintained that they couldn't actually hear the music from the noise. They were reluctant at first, but after a trial rehearsal where I set the levels on the amps and PA, everyone was commenting on their improved ability to actually hear what was going on. Since then we always rehearse at sensible levels (well, we're still fairly loud due to the drummer, but well within reasonable levels), and I've bullied everyone into using ear protection. They'll thank me later.
In this case I think my band just didn't know any better, and were fortunately quite reasonable about the matter. If I were in the situation where the band wouldn't use their common sense and refuse to moderate their volume, I'd consider it a lost cause and jump ship. It's not worth the risk to your hearing, and that kind of refusal to compromise foreshadows other problems.
In your situation I'd just let them know how you feel ASAP. Tell them it sounds bad because it's too loud. If they won't budge, do you really want to be in a band that refuses to sound good?