Hey, some quick answers for you:
I am not interested in trying to remove the filters to do that "treble mod", as the RE0 treble is about perfect and it would be screwed up.
I quite liked the Mini^3 amp with the RE0, but the overall improvement was negligible compared to the hassle of carrying a portable amp. The first day it's fun, then it starts to be annoying.
Anyway, I am not sure if I have to suggest an amp with bass boost for the RE0. Most bass boost features would spoil the balance and add unneded volume and bloat to the RE0 bass. For example, it's what happened when using the darkish and bloated iBasso T4, which detracted from the overall sound.
What the RE0 seriously lack is bass depth. The best way to fix it would be to experiment with tips. I don't have any at hand, but if I had to bet my money on a pair of tips, they would be Etymotic black foamies (otherwise considered gray, but they are pretty dark gray... anyway, you get the picture). These are wonderful sounding tips, and the only one to make justice to the deep bass Etys have. Olives and Complys, in comparison, suck at reproducing deep bass.
They also add some sharpness to the sound compared to the Olives, at least with the ER4, but it's not something I would worry about when adapting them to the RE0.
You need to adapt these tips since the nozzle diametere between the RE0 and the Etys is different. So you would have to remove the inner tube of the ER4 foamies (easy), and add insert in the foam the tube of Comply T400 tips (easy). It's not a hassle of a process like removing the Olives plastic tubes (that would have been hard).
As to durability, Ety foamies used to last a couple of weeks a pair to me. The first week they are wonderful fit wise, the second week they start to suck and are not so "full" in the ear like the previous days.
It's very likely that the midrange comes more forward too. This was another headfier's impression (jlingo) when adapting the Etys black foamies on his Triple.fi 10 Pro.
As said, tips are your best bet. Otherwise, you can try something with a mild bass boost, applied to the deepest regions. The Xin Supermini IV should apply there.