I've been in the exact situation you have been and would like to shed my 2 cents... In your OP, you stated you are going to be moving away from 2 X 12's. What you don't want to do is go through all the time and money only to be dissatisfied because of less output. IMHO, with your airspace, and for what it appears I see you struggling with, I would do one nasty 12 or (2) nasty 10's. I've heard 8's, and though you can make them bang, they just don't 'appear' to have the same presence as larger cone subs. Yes, I know 8's can work well. They just don't really appeal to me, and I listen to speed metal ;-)
If it were me, I would prolly do the (2) SA-10's ported w/ insane amounts of clean unclipped power. I'm an SQL guy. I'm running (1) Digital Designs DD9512 1.8^ft net ported to 40htz on 2000 RMS clean. I listen to heavy metal mostly. Before that, I briefly ran (2) SA-10's sealed and was NOT impressed (no output), because I have only heard ported subs for the past 12 years. IMO, don't go sealed. I know many people will disagree, maybe even you. However, if you want to retain SPL and SQL, go ported. If you have up to 2.5 net to play with and you don't care what direction the sub/subs are firing, get one really nasty 12 (or) 2 10's with as mush power as you can. If sub direction is very important, it's really hard to do better than (2) SA-10's ported, for the price ;-)