When I was at Axiom (my first time) I got to hang out in their demo room with M80's, M50's, EP500 & EP600 all on a switch box. We played around a lot and to me there was a definite difference between the 2 subs. If it were me and I had the money I would be hard pressed not to go with the 600.

On a side note, since then, there really has been a push to going with multiple subs. So now, I would be more tempted to go with 2 EP500's or 4 EP350's...all depending on room/environment.

High Gloss Cherry
M60 VP150 QS8
Open for Auditions but please don't drool on the High Gloss