Since I dont have the input screen for HR on this sub what wattage/voltage should I aim for per 21"?
Post #1 has the preliminary cabinet inputs in HR for a 21. Subject to a bit of tweaking depending on how the finished version measures. Should be close. Change the driver parameters depending on the driver wanted.
As far as the amplifier required it depends on the driver used and how they are wired. With most of the drivers I've modeled this should have about 1 to 2dB sensitivity advantage over an Othorn from 35-95Hz. It will make a lot of noise with just a 500w amp. If you were to put a pair of B&C 18PZB100's in there and plug some ports to drop the tuning a 500w amp should be a decent match. If using the 21sw152's or one of the other big neo pro 21's and you are looking to get everything out of it a 4-6 kw rated amp should get the drivers near xmax or slightly past. Of course cutting that in half to a 2-3kw amp only loses 3dB.
What I'm saying is you don't have to use a 6000watt monster amp on one of these. It is a sensitive cab so it's going to do a lot of work with even a limited amount of power.