4 ohm wiring and bridged over the two terminals will be the best setup here.
As for replacing the amp, the rule of thumb is you only gain 3 decibels every time you double power.... so going from 300 to 600 watts is 3dB.
I would personally be happy with that amp. I am running 150 watts to my sub right now and it's good... but then again I'm not as big of a basshead as most
 (got that out of the way when I was 18 hehe)
 (got that out of the way when I was 18 hehe)




