IBmack - what you've observed sounds strikingly similar to an oddity in Axiom's amplifiers (particularly EP600) that causes subwoofers daisy chained through them to drop off drastically in volume.
Awhile back someone much more inclined to electric engineering than I (Mojo) measured the input impedance of the amp and determined that it was abnormally high, which causes this effect. The fact that it went away when you switched cables means that maybe the EP500 does not suffer from the same effect, but it sounds so similar that I thought I'd at least mention this problem with the EP600 amplifiers and daisy chaining.
Jason
