You might want to try the Crystality plugin which you can get here: http://fanthom.math.put.poznan.pl/~gyver/crystality/
I just downloaded them both, unzipped them, then did (in each new directory):
$ make install
Only one of them is for xmms, I am just not sure which one.
Anyway, I noticed a fairly remarkable improvement in sound. It isn't exactly an equalizer, but it might help you out.