Well I'm not really building the system for metal, as I don't listen to that much relative to the rest of my stuff, but when I do I want it to sound good; no permanent modifications needed.
Thanks Hirsch, I'll look around. Right now I have 3 EH tubes, 2 Sylvania VT-231's, and a Ken-Rad VT-231, but I haven't done much rolling yet.
I'm contemplating just buying an SA5000 or some grado though, as that might be the best way to get the speed and impact I'm looking for.