I made my own custom fit IEMs. The isolation is considerably better than standard in-ear buds. I made them with the Monoprice 'phones, with the foam/gel part removed and custom fitted into the ear moulds. The Monoprice units were bass-heavy, and that's fine for me. I actually have a spare set of the Monoprice units, just in case.
Sometimes, I just put in the ear moulds, it helps reduce the sound levels.
In terms of the IEM audio mix, I find that the bass feed has to be loud enough without it causing the drivers in the buds to distort, so some time spent in getting the mix from the desk into my feed, more by reducing other inputs rather than turning stuff up. I also have to factor in my hearing and the effects of my age on my ear's frequency responses. So it is good not to share my AUX mix with anyone else.