You would be better to keep both subs and place them on opposite walls centered. While the output of the 600 is greater than the 500, you not going to find a huge difference. In that room you need multiple subs. I've pretty muched owned all of Axiom's subs at one point or another and I don't think your going to see a huge difference.
Placement is key, so not sure you have experimented. Maxing out the volume is not the answer.
I currently have an EP600 and twin EP350v3's spaced out around my 8100 cubic ft room.