I have inherited another set of ER-4Ps from my nephew (which I bought for him for recording work a few years ago). They've got distortion, presumably from the earpieces/transducers themselves - there doesn't seem to be any issue with the cable (which is enormously frustrating as I just sold a set cheap because of cable issues.
If there's distortion, could it be related to the filters? Excuse my ignorance folks. :)