Alan really is very well spoken and that is a great read. It really got me thinking, so I decided to so a test. I went to my theater, and played a song that has nice "tight" bass. To limit my findings, I listened in stereo mode only (with my subs providing the bass). It sounded really good and "tight" and "full". It probably does because I calibrated my subs, and have a BFD to smooth the curves out.
Next, I turned the subs off. The "tightness" was still there, but the "fullness" was gone. I could still hear the quick attacks, but there was nothing behind it to fill it in.
Next, I set the mains to large, subs still off. The "tightness" was still there, and there was more "fullness", but not as much as with the subs.
Next, I set everything back to original, but brought the subs up 6db in the AVR. The resulting sound is would I would consider muddy. I loss the sense of "tightness".
Having said all of this, and after thinking about it for a bit, I drew an analogy to drawing a picture (people are always asking for terms to describe the sound, and in my mind anyway this a good representation). The M80s are drawing a nice sharp outline of a picture. The picture has a very nice sharp edge all around it. The problem is, that the color is weak, or not there at all. If you decide the paint the picture (to give it fullness), the sub can do this. If the sub is set too high, it colors the picture outside of the lines and covers up the sharpness and beauty of the original sharp drawing. If adjusted correctly, it colors the picture perfectly, and does not "go outside the lines".
Some posters have complained that the EP600 is not as tight as other subs. My speculation is that the sub may be set too high, or not calibrated correctly. This would result in the sub painting the picture a deep red, both inside and outside of the drawing. A weaker sub would color the picture short of the lines, in a nice warm pink. The resulting picture looks better than the black and white sketch, and certainly looks better than a deep red all over the place, but is still not as good as it could be.
I feel that the EP500/600 should be treated as a deep red paintbrush that must be properly applied (and not over-applied) to the overall picture to result in a masterpiece worth hanging in every home.
Thanks for the post, and thanks to Alan!
