With 2 EP500s you won't need to upgrade. To go beyond that much power you would have to spend, I would say, double the cost just to get a marginal improvement over them, based on reviews of subs that extend lower and have more SPL.

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