No, did NOT verify.So you set a HPF at 20hz ?
Did you verify the box is actually 21hz tune, they seldom come out perfectly tuned to what is planned........
If so, then I would also recommend a stack of PA460`s...........tuned to the high 30`s,low 40`s like a pro sub, or better yet a pro sub !!!!!
4 PA460 horn cabinets ?