"Mark Zarella" seesigfile says...
Indeed ... I think I should look into a parametric EQ for my system. For
some reason, my speakers reproduce some (literally) painful high
frequencies ... it seems to be within the third slider from the right on
my EQ (10k maybe?), but taking that slider all the way down affects the
rest of the audio, and it is difficult to find the balance I want.

Sometimes the tonal quality of the speakers themselves or their installation
is very difficult to overcome with an EQ. You may want to give the
installation another try,

Ummm ... I already cut my door panels ... are you suggesting that I
actually move the tweeters somewhere else?

or even try throwing a different set of speakers
in there at least for testing purposes.

I would need access to some to test.

What speakers are you using anyway?

I am using the Diamond Audio M661 component speakers.