here are your options...
get a new amp that puts out 800w RMS either at 2 ohm or at 8 ohms x1
get DVC subs...dual 4ohm voice coils would either present a 1 ohm load or a 4 ohm load depending if series or parallel... then you could run 600 @ 4ohms x 1 to them...but you'd still be underpowering them...