Going to get so much crap for this suggestion, but I'd try the 2009 Kicker Solo classic new on ebay. They are kind of pricey but they sound shockingly good, clean and articulate. Plus easily handles 750rms and is the exact airspace you are asking for. These sounds nothing like your typical muddy kicker and are built much better with better glue, spider, and materials. Kicker recommended .88 BEFORE displacement so you can get away with a tiny sealed enclosure.