Not sure if this is causing your problem, but it sounds like your running the sub a little to hot. Axiom recommends starting with the sub volume(gain) between Min. and 9 o'clock, which would be around 6.5 to 7 o'clock position. In addition, if your receiver is at +1.5 dB that would seem to be to much.
Does your receiver have the built in microphone setup? Or, do you have a Radio Shack SPL meter? I bet if you calibrated everything to 75 dB's you would find that the receiver sub setting would be at more like -6dB's and/or you would need to turn down the sub gain.
When I first got my EP500 I had the gain way to high, and when I ran some test tones, especially at very low freq's around 20 hz, the Woofer was really traveling in/out a lot, and producing a clapping like sound, as Wid described above.
I found I was running everything way to hot.
