I'd suggest an XBL^2 sub, for numerous reasons.
Primarily though, you are looking for a sub that can maintain its composure, even with rapid-fire high-impact double bass hits, along with whatever other bass material will be going on (metal, you know)...
XBL^2 technology fundamentally eliminates intermodulation distortion, where other technologies (save for JBL's DDD technology) can't do that... and IMD is the primary audible distortion present in a loudspeaker.
It does so by virtue of flattening the BL curve like a table top, so as the voice coil and cone are moving in and out, the BL value (motor strength) remains constant - where with traditional motor architectures, this value fluctuates, decreasing the farther in either direction the sub cone moves - and as it changes, so do all the T/S specs. //content.invisioncic.com/y282845/emoticons/eek.gif.771b7a90cf45cabdc554ff1121c21c4a.gif
In fact, it cleans up the sound so much, that many enthusiasts have commented that it doesn't "sound" as loud - because it no longer contains all the violent awkwardness of a train wreck as most subs do as a result of IMD... it's not as violent, it's realistic, accurate.
It also happens to have the side effect of increasing your excursion capabilities... so, you can reach monsterous excursion levels... read: monsterous output levels. You do listen to it loud, don't you? //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif