Notice I didn't say bottom heavy I said peaky. Avalanches peak low as hell in any decent size enclosure, but brahmas and xxx's tend to peak around 40hz unless in an oversized enclosure...... this is just from personal experience. Atlases peak even higher in recommended enclosures, but its not a hard peak at all Atlases are some of the most flat drivers I've heard ported, in car. You can ask timb2169 about some low end though, he had an avalanche 15 in something rediculous like 5-6 cubes net tuned to 26hz in his car and that thing just tore up sub 35hz notes like nothing I've ever heard in a car. Hell 20hz wasn't even a problem. XBL^2 is definately good for its purpose, but weigh in all of the losses of it too. Honestly look at how huge XXX, Avalanche, and Brahma motors are compared to that of an SX, DP, hAvoc, BL, Mag, Q ect..... and look at the higher motor strength of the conventional motors. Not to mention that all of those drivers sound great.
pimpdaddyq - All of mine are just loving the beating they get. I've switched a couple of times between two and four. I have a 10 cube box to 31 for two that just dominates the low end and a 12 cube 32 hz box for four that is just louder then shit.....were talkin' 150's with 2500 watts. I'll eventually build a new box for four that is 16-17 cubes to 32 with 220sqin of port area. I'm gunna have two drivers and the port firing back with two drivers inverted firing up. I think that will be the perfect setup for my vehicle. I'll be sure to post some pics when I get it all done.