Blew out Kicker sub ?

I installed a kicker 10' comp vx 2ohm sub (600 rms) to a mono 400 watt kicker amp (zx400.1). The sub blew out in a little over a month. I was told by Circuit City installers that my amp is underpowering the sub and I should get a more powerful amp. Kicker told me that the amp is fine, but I probably had the gain turned up too high. How am I supposed to know what levels to set the gain, bass boast, and cross overs? Also, do you think It's better for me to go with a more powerful amp like the 600 watt or 750 watt kicker amps?

Thanks for all your help!

I couldn't find anything that those guys postes in the past. I figured out that the gain is supposed to set at the radio's pre-out. but my radio is the pioneer dehp6800 and it's 2.2v. I set the gain and I can barely hear anything. I still can't figure out where to set the crossover and bass boost. I just don't want to blow out my sub again. The sub is 600 rms and the amp is 400rms. Is it better for me to get a 600rms amp?