I think I would go for the extra low end the 800 offers and then add the 500 to smooth things out through out the room. Just run the 800 with your present subs until you can come up with the rest of the money to get the 500 later on.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70