Re: High output subwoofers.
You need to hear the other two, the T21 is a solid step or two down from either the M421 or Danley TH812.
One possible reason to go with the Danley TH812-the likelyhood of driver failure is spread out over less expensive to replace/recone 12" drivers vs the very expensive 21" drivers used by McCauley or Adamson(or anyone else).
That said, I'd also look at the Danley TH221, as it is pretty close in sensitivity to the TH812, but has an F3 of 22hz. Depending on what low end cutoff for the system is desired, dialing a group of 12 TH221 with a high pass at 25hz instead of twelve TH812 set with a high pass of 30hz may have more impact at nearly the same SPL levels because of the extra bandwidth at the low end. Given that it's outside, this may be "more better". ;>)
Best regards,
John