hmm
i am familiar with both subs, and have seen them compete, plus I've owned an 18" x.. myself.
I would say the x.. for a few reasons:
1. showoff: one sub being louder than the other guy's two 12"s makes you look cooler
2. lowend: the XXXs may be SPL subs but coupled with the xmax of 68mm total p-p movement you can get a lot of lowend (but the magnums could possibly compete...)
3. two magnums: 500 watts each - they don't want much more power, you have too much. unless you are competing.
4. 12" x.. - 1600 watts my ass! these subs laugh at 1600 watts, and beg for more. definately power hungry and will more than likely sound better.
5. xbl^2 technology - much better sounding bass, even with an 18" x.. ported in an 8 cu ft box tuned to 33Hz rivaled my 15" brahma sealed in a 2.5 cubic foot box in SQ. xbl^2 is simple, but amazing technology, you WILL notice the difference with it, you WILL think "this kid has mushy sounding subs" when listening to other systems.
6. smaller box capabilities (but I strongly suggest NOT sealing this sub, the output will be stopped far below 140dB if you seal it.)
if you get the x.., I suggest a big 4 cubic foot box tuned to 38Hz with about 70 sq inches of port. if you get the magnums, I'd suggest 1.2 cubic ft sealed per sub.
the x.. is definately a sub that won't break from abuse easily.
