I'm very lucky that the bands I play in - thus far - don't play exceptionally loud. I've had very good luck with ER15s for years. I haven't found that the ER20s roll off too much high end, but that they simply roll off too much of everything. It's still relatively flat, but it sounds like it's in the next room. For that reason, I prefer the 15s.
Human hearing is a bit funny in that, as we suffer hearing loss, we lose sensitivity to certain frequencies when they are at lower volumes, but we become more sensitive to the same frequencies at higher volumes. For example, those of us who have mid-frequency hearing loss will have trouble hearing people who speak softly, but loud mid-frequency sounds such as guitar amps will be more painful to us that people without hearing loss.
For me, the 15s seem to balance things out in a way that enables me to hear vocals clearly without mids getting muddy the way they do with more attenuation. Everyone's ears, and degree of hearing damage, is different of course, so it probably pays to experiment.