sure, for electric/electronic instruments, but: horn players play 'acoustic' instruments --- e.g., a soft trumpet part is still loud, acoustically, if your ear (or a mic) were to be placed 4-6 inches in front of the bell of the instrument. and if they play normally = loud!
per the OP: the IEM threads convinced me that i was not getting the
proper seal required for bass players. i'd be in the crowd suggesting that custom molds are nice for comfort, but that the critical
seal necessary for bass can be attained with any tip that actually
seals out the stage environment! once i found the tips that worked for my ears/canals = great in-ear mixes no matter how loud and crazy the others were --- and that's with three horns (both bands). the proper seal made all 7 or 9 pieces almost disappear on stage (like they were playing in a house way down the street!) unless i turned up their fader in my monitor mix. the best seal will let you
turn things down.
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TL;DR: once i discovered/had
a proper seal: i was protected from everyone else + i had the ability to make a mix that was low volume with as much bass as i wanted/needed. the
proper seal made all the difference!
good luck with your IEMs, joe!