Some of the answers are above. There are two answers. The first is that if it is a professional band in pre-production, you need to have a discussion between the band and the monitor engineer about stage levels, and whether you are using in ears or wedges. In ear monitors allow the stage level to be much lower, and are the answer for professional bands.
For most people, the answer is to reduce the level of the drummer, the snare in particular, and make sure the other instrument players have their amps pointed well.
Drummers may not want to dampen their kits in rehearsal, as it means they don't sound as good, but if you are not playing in public or recording but rehearsing, a snare drum, (or whole kit) should be dampened, but that's not discussed by drum teachers! A good drummer will hit the skin hard in the middle of the skin, and it will be loud. You can play quietly, but it is much harder technically, and it doesn't make the same sound. Our ears are especially sensitive to 3-5kHz frequencies, which the drum produces, the guitars also work around here, as do keyboards and vocals, so the mid-frequencies are too crowded. Toms have lower tuning and dB level, so snares are the main issue. Your drummer can put a T-shirt or Tea Towel over the snare; you can buy some gadgets that will do the job; you can put a wallet on the snare; you can tape wads of tissue onto the skin with gaffer tape. Moon gel might reduce ringing but will not reduce the dB level much. You can buy pads to put onto the snare. Once the snare is quieter, the rest of the band can turn down. Most bands rehearse above a level that will damage hearing, and snare and guitar are the main issue, due to the ear's sensitivities to those frequencies. You can also put pads on the bottom heads of drums to make them quieter.
Then, as someone else said, make sure that guitar amps are pointed correctly. Guitar amps don't work for the first metre of so, very efficiently. So the guitarist has to be standing at least a metre from their amp. Most guitarists wrongly put their amps on the floor, pointing at their ankles, funnily their ankles do not hear the sound very efficiently, and the band member opposite gets the full force of the amp. Lean the amp's speaker back against the wall so it is angled towards the guitarist's ears, or ideally bring some wooden blocks so that you can put the amp in front of the guitarist's pedals angled up towards his head and away from everyone else. Some guitarists will have a loud valve amp, like a Fender Twin, and say the sound won't be right unless turned up to a certain level. The answer is to get an impedance load device or preamp pedal, to change this, or a smaller amp for rehearsing (if pro) or for gigging (if not pro). Or rehearse with a compromised sound.
The low E on a bass guitar is 41Hz, so the wavelength is 8m long, meaning to hear it properly they need to be 4 metres away from the amp. In practice in most small rehearsal rooms that means bass amps should be placed behind the drummer, and bass players should stand at the other end of the room to the amp.
The next thing is to start your rehearsal by mixing the band, so get the drummer to plan, get the bass player to join in, with his guitar on full get another member of the band to turn the amp up, till it sounds right to everyone. Then get the guitarist to do the same. Then keys and everyone else. Get everyone to agree on levels. Then get the drummer to play and mix in vocals. Add in bass. Then add other instruments. Make notes of levels, and use the same levels each week.
Onstage live is a whole other issue.
