The opening is more just consideration than anything else.
I think to good 15" Subs would be better than 12s, but the Mono 12" or the PC2Ks should be a good start. I think I would lean more towards the Monoliths before considering PSA or Rythmik.
OP should consider getting the best single Sub first, then saving for a second matching Sub, if possible.
Budgets are easily blown, but as we all know, good Subwoofage is an investment. Done right the first time, it can save a lot down the line.
That said, stepping up to the Monolith 15, or if he can stomach the cost, the Monolith 13s.
Right now the Mono 15v2 is on sale 1200 each.
Otherwise, the 13s for 1549 right now (slight sale there too.).
I still don't think PSA is worth it. Those pro drivers in such small cabinets... I still don't see how he is making 12 and 14Hz performance claims.