Judging by the picture, it looks like the grills were left on most of
the time and they kept the dirt off the area that is lighter than the
rest of the cabinet.

I would try cleaning the front baffle with some sort of wood cleaner,
such as orange glow.

If it really bothers you and you feel brave, you could remove the
drivers, mask off everything but the front baffle, and strp the finish,
then restain. Even if it's not a perfect match to the rest of the
cabinet it will at least be even on the front.

A word of caution, use the stripper and then sand with very fine
sandpaper, in order to not strip the veneer. If you are as careful
with them as they deserve, I doubt yu'll have any problem.

Try the cleaner first.