I have had the same amp for over 10 years or something (mosfet california gold 2 channel 400W) and hasnt failed me. It powers my 2 10" 250W rms subs in parallel bridged very nicely.

I know mosfet is a good thing but what exactly is it ?

I believe it's a high quality digital switch.....used to amplify digital music signals. It's the heart of your amplifier.

metal-oxide-semiconductor field-effect transistor

The digital alternative to a valve amplifier.

http://en.wikipedia.org/wiki/Mosfet