You'll get plenty of low frequency response with the 500. I think the 500 will work in a room up to 5,000 ft^3 provided you are looking for clean bass to 20Hz. Slimpikins enjoys clean bass out of his 500v4 and he's in a 10,000 ft^3 room with Bryston Model T powered by two, 2,000W amps that are fully cranked! I have an 800v4 in my 4,200 ft^3 room and it's turned up less than half-way on the dial. If you get a sealed Axiom sub, ask Ian if he can NME it for you.
If you need right down to 16Hz for the lowest note of a pipe organ, then buy the 600. Ask yourself though how low can your room go and still be able to support clean sub-bass?
If you are only interested in good bass response at a single listening position, then one sub is fine. If you need flat response at multiple positions, you might want to consider up to four subs.
As for how far the 100 has to be away from the front wall, I don't own one but I can tell you hometheatreshack recommended a 3 foot minimum to tame the bass. Other reviewers had a tough time with how much bass it threw out. As for depth of soundstage, I have the back of my M80v2 3 feet out and i can definitely perceive depth but I want more. I am considering upgrading my 80v2 to M5 which I know will give me a more refined soundstage.
