With typical IEM, I get about 25% use of volume pot with E12A. I enjoy the sound signature but wish the low gain were even lower. I haven't heard the JDS C5 but I suspect it's a leaner sound signature than E12A and might have better control over volume with typical IEM. Someone who's heard both will have to confirm, though.
It's been mentioned before, but I'll post it again. Headfonia compared E12A vs. JDS C5 here: LINK.
Listening with earphones straight out of X5's LO? You know volume on LO is always on max, right?
I made that mistake once and almost damaged my hearing...