if the subs are 8ohm and not 4ohm they will I believe see 500watts each.
At 4ohms bridged subs would only be a 2ohm load, unstable on your amp.
400 RMS is kinda high unless you spent $$$ on them but anyway... with a mono amp you get what it says 500watts. now im not 100% sure but bridging the 2 speakers by taking the +/- from one sub and just putting another wire from it to the next subs +/- you will halve the ohm load but keep the wattage load.
I think. I mean i run 100watts into 2 75-125watt RMS subs bridged and they still pound so its not running at 50w or they wouldnt even make a sound.