Out of curiousity, if you only had two choices and they were between those listed in the title both running in the SSA recommended Optimal Sealed box's, being pushed by an MB Quart DSC-2000.1d, which would you choose?
Xcon 15" Optimal Sealed box:
Sound Solutions Audio - 15" XCON Optimal Sealed Enclosure
Icon 12" Optimal Sealed box:
Sound Solutions Audio - 12" ICON Optimal Sealed Enclosure