It's better to have a 600 watt amp running your 500 rms sub than a 450 watt amp because you get more clean power. Use the higher power amp and turn down the gain, as apposed to maxing out the smaller amp and have it clip. Also, I wouldn't worry about running 600 watts to the sub, because most music isn't loud enough to create that 1000 watts max out of a 600 rms amp. Just listen for too much distortion and adjust accordingly.
