If its a fun Jam, everything goes-Its a time to be creative but have some restraint for the other band members to be able to like-wise "let it out"
If its a Jam for a gig preparation, Hold back on your improvisation solos, just work on the band being tight.
If during a gig, especially a paid gig, leave the volume and EQing of the band mix to the trusted soundman-he can hear much better from where he is, and listen to his advice when he tells who to turn down, up, or whatever.Don't argue just do it.
Just make sure your amp volume is comfortable for yourself(as in you are able to hear yourself), and leave the rest to the monitor speakers on the floor for the drummer and bass and singer as their reference markers to hear the cues involved in the song.
Sometimes I even turn my amp away from the audience, so that I won't mess with the speakers volume mix...(my amps are always mic-ed by the way)
For me, the most important aspect of the band is the singer.Support him, not compete.Make sure he/she is comfortable.Some bands can have killer guitarists and instrumentalists but if the singer is struggling with diction and unable to convey the emotion or message of the song, its really pointless.
During fun jams at pubs, where there are impromtu jammers, its hard to be aware of how loud you really are.(its all heineken's fault lah

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Fun jams in pubs should be relaxed, and you should make sure that every jammer in the band is being showcased, not only your guitar skills and shred or whatever, even the 55 year old off-time bassist should be having his fun and deserves to be heard.Hold back and smile and have fun.
20 cent