Alpine MRP-M500 Mono subwoofer amplifier 500 watts RMS x 1 at 2 ohms at Crutchfield.com
I would suggest this amp over the 4 channel that you have there. It will give you more power and it should sound better, just run the subs in series.
Just make sure you wire the amp for 4 ohm instead of 2, this will give you the right amount of power.