I do agree with this, mostly. Again, tuning makes a HUGE difference.
Even turning my subs in a different direction totally changed the "muddiness" double-bass kicks have in metal music.
But yeah, get back to us on that amp, and as far as size, a 10 or two should be great, and for metal, I'd just drop them in a sealed box, more "punchy" and crisp, not as obnoxious. Ported boxes will make it muddier if it's not tuned right, and with a prefab box, that's easy to have happen.