I think the Pioneer 7500MP would be a very good choice. All kinds of flexibility.
I also think you'll need two of those amps to power two of those subs you gave in the link.
the amp 501bd = 300wrms X 1 @4ohm
the subs = 400wrms dual 8ohm each. Just looking at one amp per sub the dual 8ohm is fine, you wire it for a 4ohm load to the amp which will give you 300wrms. And you want to put two subs on that amp? Not knowing the 501bd personally, just looking at the numbers,I think you'll be way underpowered.
Maybe look at some other rf subs that don't require that much power to move them.
And the diff bw the 800s and the 800x?
800s = 2channel, 200wX2, 800wX1
800x = 4channel, 100wX4, 400wX2 (this is the kind of amp you would need to power the four interior speakers, 4 channels, 4 speakers)
Read the specs, get to know your stuff before you spend all your money
Gene.