A sub doesnt "peak" at 400 watts RMS. Peak and RMS is completely opposite.

If you have a sub that is 400 watts RMS, you probably should get an amp that does more than 300 watts. That will decrease the chance of you clipping the signal. You also have to look at the impedance and make sure they match up.

What sub are you powering? What price range for an amp are you looking at?