I'm totally serious about this - until I got hooked on the idea of getting the Mystery amp, I was waiting for this company to finish up testing on their 24" subwoofer!
http://stereointegrity.com/index.php?id=61 
I've already got a 21" Maelstrom which is nearly "flat" down to 10Hz (no joke!) in an oversized box I built for it. I was looking to add a second woofer to my home theater to add more pressure to the room, as well as balance out the rooms frequency response. I've already got another 2 kilowatt amp ready...I just need the funds!
Funny twist in the story...since wanting this speaker, I've gotten my Zen amp tuned up and upgraded, purchased my MG944 speakers, and added an Oppo BDP-105 Universal player...so the MG944 and 21" sub are now both using the Oppo as a 2.1 setup that's just fantastic! So either way, I'd still have to figure out a way to "share" this sub between my 7.1 system and my 2.1 system. After hearing and feeling a 21" sub filling in under the MG944, I don't think I can go back to a 2.0 setup! :D