Check this out...
They will handle 400w rms... on a sinewave, not music. They will be getting less power on music.
So, do this. Get a -10db 40hz test tone ready. Set your EQ flat, Sub crossover all the way up, no loudness, no bass boost, bass knob if you have one maxed all the way up. Disconnect your sub wires from the amplifier. Hook up a DMM's leads to the positive and negative speaker outs on the sub amp and set the DMM to A/C Volts. Turn your head units volume up to its max undistorted volume and play that 40hz test tone. Start turning your gain up until the DMM says 34.64 or rounded to 35v. 34.64v at 2 ohms should be near 600w. Now hook everything back the way it is supposed to be... set the crossover to like 80hz, turn down the bass knob, hook your sub back up, turn the head unit down.... Play some music and enjoy. You will now be closer to 400w RMS on music. Use a -10db 40hz test tone.