actually its depending on the type of metal...
if you have a dense mix like say... cannibal corpse.. then you want lots of mids.. as Alex Webster states..
then on the other hand... Dave Ellefson says he scoops his EQ because in thrash metal the guitars has LOTS of mids.. thus making it pointless to try and cutthru with the mids..
There are many different bass tones (and guitar tones) in thrash metal.