[quote name='Phil Starr' timestamp='1445708814' post='2893668']
A possible solution.
Guitar amps are more directional than bass amps and unless he has a decent amp stand his amp will be pointing at his knees. This means he will only be hearing the bass from his amp and not the screetchy high notes. He is probably eq'ing so it sounds good where he is standing and he is turning up so he can hear clearly when well off the axis of the speaker. Explain politely you think this may be happening and ask him to lean the amp back at rehearsals so his ears are in line with the speaker axis. If he balances the sound then it should sound better for everyone. Then he needs to get a stand so the speakers are pointing straight at his ears at gigs too.
I've used this tactic successfully more than once.
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Possible alternative way to the same end: Buy/lend him a wireless rig, so he can go walkabout, and find out how crap he sounds for himself.