You could try wiring them in series for a 8 ohm sub but you'll only be getting about 150 watts out of that amp at that load. The SQ probably will go up but SPL will go down a bunch as well.
What kind of enclosure are you running that 15 in? I'd say the enclosure is every bit as important if not more to SQ as your amplifier.
If you are running a sealed enclosure you may very well be overdriving that sub at that system's resonant frequency especialy since that amp is rated 100 watts higher @ 2 ohms(600 RMS) than your sub(500 RMS). Thats one of the drawbacks of sealed boxes: Lotsa excursion at resonance.