I can't see how the distortion is less in the Sony with a 120W amp, but it is possible. We may be zeroing in on a possible problem which is coloration introduced by the enclosure. Back when these EP600 subs first came out there was lots of discussion at other forums that the size of enclosure merited more bracing. It wasn't an issue with the 500s. I believe there is only one large brace in the EP subs.
While I have never heard problems with my 600 enclosures, with my ear running up and down the sides, it is possible that something might be off in your sub. You mentioned how the sound changed when on its side and you "beavered" it..hmmm...never mind.
Just a shot because I still believe the main culprit is room acoustics and speaker integration, but I'm curious if lying it on its side and putting weight on the middle does much of anything.
Let me know how track 3 "Take Five" from this CD sounds.
