I have had a tough time narrowing it down to a decade or a genre. The very worst recordings seem to be from the 70s and 80s -- as do the very best recordings. It's the midrange level that matters more than the high treble.
Here's a good example -- B.B. Gabor's anthology just came out on CD recently (Gabor was very popular around Toronto). The CD sounds great on my car system (the entry level system in a Dodge Magnum) but sounds harsh on my M60s.
EDIT -- I guess I could have picked an album that somebody not from Toronto might have heard of... but it *is* a good album anyways
I have very few recordings that sound bad on the Axioms, probably because I have generally owned pretty flat speakers (Rogers LS3/5A's being the exception, I guess, with their famous BBC dip
) so the albums probably would have been tossed or given away long before I bought the Axioms.