"David Bash" wrote ...
Anyway, it seems that every CD I've played that had been
in the room sounds worse than it did before, with most of
the noticeable loss manifesting as muddy, dull midrange
and clipped high-end. In other words, a huge loss of dynamic.

My question is: is this all in my head? Can CD damage due
to excessive heat manifest as a loss of fidelity? Hopefully
someone here who has expertise in these matters can help
eliminate my fears.

I'd strongly suspect that if there really IS a difference, it is
in your playback equipment, and NOT the disks. So play one
of the suspect heated disks on somebody else's high-end
system and compare to a new copy. Should put the doubts
to rest.