so you are looking for a sub that is at least 1600wrms, dual 2 ohm or quad 4 ohm coils, small airspace requirements, under $500? If you are only going to get a sub that is 800wmrs then you dont bother wth both amps. Its also ALOT safer to run a single amp to the sub unless the gains are matched EXACTLY. That doesnt mean on the same mark on the amp either.

You arent asking for much are you?

my advise would be a single sub single amp setup. With your requirements though I would suggest either an Kicker L7, or a RF T2