in my opinion, you shouldn't have to dial a fundamental tone differently for different speakers. if we mic a real amp/cab, that signal is going to all speakers, IEMs, etc. you don't use a separate amp for your in-ear tone.
what people may not realize or try is to EQ each speaker send specifically for the speaker. FOH always EQ's the mains, stage monitors are always EQ'd to prevent feedback (and hopefully to sound good). there's no reason an IEM send shouldn't be EQ'd as well.
for example, here is the curve i use for the Shure SE215:
View attachment 51340
these are the $99 IEMs, but i prefer them in most situations because of how predictable they are. in my experience, having way too much Low Frequency in an IEM send is what kills the experience for many. yes, we want to feel the bass and all of that, but the driver/speaker is millimeters from our ear drum - there isn't enough space to feel anything there. so i dump a bunch of lows. even after this major cut in lowend, i still feel the fundamental and great bass tone from either my guitar, or a bass guitar, kick drum, etc. you'll have to experiment with how much to cut.
once this low end is out of the way, everything else above it feels like it opens up suddenly, with much more clarity than before. i then take care of the most irritating mid frequencies that i hear - set a small Q, then boost and move around till it's the most irritating, then cut it. for me it's around 250 and 2k on the SE215. and with this particular IEM, i feel there isn't enough high end, so i boost the high shelf to taste.
i don't have a picture of it on me now, but my curve for the Westone UM30Pro is completely different. i think i leave the high end alone, don't cut as much bass, and the mid notches are way different. i actually have more trouble dialing in the UM30Pro than the SE215. they both end up sounding great to me.
a thing to remember is that these are
monitors so we can hear what we are doing clearly, and perform well. i feel they shouldn't be used to "sound like a recording." listening to music and performing music are 2 different things.
dial in your guitar tone to sound good on your speakers. then EQ the other speaker systems to balance out their own voicings, and make THAT sound like your original tone, instead of redialing your tone.
and yes, a touch of reverb or room sound goes a long way for the entire IEM mix. just a bit.