I just had a set of MB Quart Discus series components installed in my new
Mazda 6 sedan last week. I have read that Quarts have a reputation for
exceeding brightness in their sound output. That's my problem. The
clarity
is outstanding but the overall sound is way too bright even with the
treble
completely cut out. just unsatisfying no matter what I do.

You didn't listen before you bought?


Are there any adjustments that can be made to the components to get a more
natural sound? It's driving me nuts! I do have them still running off
the
stock head unit's amp, not an external amp. My next thought is to buy an
amp
to run the speakers, but would that cut down on the brightness?

Not significantly. Try the typical off-axis mounting and covering the
tweeter with foam or something. At this point, you may instead want to get
different speakers rather than go out of your way compensating for these.