I'd do four drivers per channel for a 4 ohm load on the amp, and have them in an isobaric configuration. You'll have the same output capabilities as just two drivers, but half the enclosure size, a lot less harmonic distortion and double the power handling.
If this is strictly for music, I think this is probably the best way to go. You'll have plenty of output being that you have a 500 watt amp pushing each "quad" of subs, you'll have better detail (cleaner too) from the isobaric loading of the drivers, very tight, punchy, accurate bass and the ability to have smaller enclosures (half the size of an enclosure if you only ran two drivers).