First, the biggest improvement will come from getting the position of the sub optimized left-to-right, and front-to-back. Try the
sub-woofer crawl to help with that.
The EP500 has the cone a pretty good distance from the floor. That'll stop the biggest part of the boundary gain, which also happens when the sub is near walls. While putting the sub in a corner will seem to boost volume, it doesn't do so for every frequency equally, and can lead to boominess.
If you do want try raising the sub off the floor (remember the ideal positioning of a sub is in 3 dimensions, not just 2), you can try a
SubDude or two. The ultimate is to place the sub on top of a bass trap, to not only defeat any boundary gain, but absorb the energy which builds up into the floor to ceiling room mode. ASC makes a
SubTrap to do this, but it is a little expensive, and you'd need the 22" SubTrap to fit the EP500 on top. So I'd recommend starting with a SubDude, which is relatively inexpensive, to see if it offers any improvement.
