I don't understand why the new Infinity Kappa subwoofers are no longer popular. People migrating from the older Kappa Perfects should check out the current Kappa 120.9w and 100.9w subs. I have an Infinity Kappa 120.9w running in 1cu ft sealed box right now. It's a very tight subwoofer with a good SPL capability. The 120.9w has good qtc in a reasonable size sealed box, good sensitivity, low inductance, and 17mm xmax. I suspect that the 350watt RMS power handling is an understatement. For the price of $130-140 that these are usually sold online, they're a good bargain.
Having said this, I would recommend the OP also to check out the new 2012 Alpine Type-S subwoofer. An Alpine engineer was posting on
DIYMA that this is supposed to be a good SQ subwoofer, with the new Type-R being more of a SQL subwoofer.