A few thoughts :
The EP500 is better than the 350 in two ways -- one is that it has more power, but the other is that it also plays deeper, more like the SVS. The EP500 looks to have a flatter frequency response than even the 20-39PC+ but is probably more like a PCI in its ability to fill a much larger room.
Truth is that the pci would be fine for your room. It's a great sub and probably the best bang for the buck in the industry.
If you spend more $$, SVS and Axiom offer different things for your $$. The EP500 will give you flatter and better response in the lowest bass (the SVSs roll off a bit at the lower frequencies); the PC+ will probably play louder and/or handle a bigger room if you move in the future.
If you don't want to choose, there is the EP600 and the PC-Ultra, but you pay for the privilege
For your room the EP500 is probably the best, and the pci is the best for the $$.
