Just checking out the MTX site to see the specs of the Thunder6500D. I see these specs:

Specifications
Power 12.5 VDC
2 Ohm Load 500 w x 1
4 Ohm Load 250 w x 1

Power 14.4 VDC
2 Ohm Load 775 w x 1
4 Ohm Load 450 w x 1

Which is the right one to follow? What are the differences between these?