First, lemme explain to you the power of your amplifier.....
300rms Watts at 4 ohms
500rms Watts at 2 ohms
You would require a 4 ohms dual voice coil subwoofer (means you connect your woofer at 2 ohms net load) to get max outta your woofer of 500W rms ratings (simply D4 woofer).....
Now, in subs you have some options.....
Cerwin Vega Vmax 124 (500rms Watts)
Alpine Type R 1242 (500rms Watts)
JBL GTO 1214D
Kenwood KFC-XW1224D
More options are also available but i can`t recall them right now..... But look for something in the rms power range of 500Watts and a D4 or single 2 ohms voice coil.....