I have 3 pairs but none are being made currently. The best is Nelson Reed 1204s. Uses 4 12" Volt drivers a side , 2 in front 2 in back of a 4 or 5 CF cabinet. Will do 120 db down to 20 Hz supposedly, I haven't ever driven it that hard but on the 1812 the cannon shots are something else. Kavi Alexander use to use them in his monitoring system at Water Lily, may still. The older Hsu woofers are also good, I have the 10" and 12" ones. Just a tube with a woofer in one end but worked well.
Searching for passive subwoofer...
I know that I can get passive one searching pro-audio shops, but is there any high-definition ones available?