Jay, you need to describe your room to answer this question correctly. Multiple subs placed correctly in a room will even out the low frequency in a room for more seats, hence, less peaks/nulls.
As htnut pointed out, calibrating multiple subs in a room can be tricky and time consuming, however, if done correctly really helps the experience for everyone.
Once we know more about your room characteristics (size, shape, openings, etc..) it will help us give you better feedback. The 500 is very powerful and often placed near a corner furthest from a large entry into a room will knock your foundation down.
I had the same question about a year ago for my large 8,000 cu ft rec room/HT. I had an EP600, but wanted to experiment with multiple subs to see what all the hype was about. Do I go with two 350's and have 3 subs space out around the room, or one 500 on the opposite wall from the 600.
After working with Amie and Ian at Axiom, I went with the twin 350's. Calibrating them was somewhat interesting as the 600 sucks the dB levels from them once they were all running togethor, long story.
Currently, I moved one of my 350's upstairs to go with my new m22's, so only have two subs downstairs now.
