FTW-21s are definitely something, but they pretty much have to be used in EQ'd alignments with tons (3kW @ 4 ohm or greater) of power. So you'd absolutely need to factor in higher-end amplification and equalization too. And honestly, it wouldn't necessarily be the most efficient use of money, although it'd be really cool. It's just that as budget goes up, you could probably do six to eight of the Dayton or Stereo Integrity 18s sealed and a lab gruppen clone. Multiple Sealed Subs are pretty much the "ultimate" in infrasonic performance, while the additional sources serve to further smooth out midbass performance.