If this clarity was heard as relatively normal distribution of harmonics for a bass, until the onset of perceived error in sound, it may be that he has been chasing a sound that has been influenced by previously steady changes in hearing for some time. If so, a recent sudden change definitely must not be ruled out.
The bit that interests me is that he says familiar music tracks still sound ok, but that might mean he trusts them, and as with placebo effects, trust is a big part of perception and result. And that could cut both ways, so maybe if this is overexposure to the sound, a rest for a week might help. I have often found that a week's rest can do more than a three hour practise to improve my sound.
As of this point I'm out of my depth. I considered suggesting an exorcist because dubious funny is all I can think of now.