Hey there - sorry I haven't responded sooner. I was in a car wreck a couple of weeks ago and am trying to catch up.

The III is extremely well behaved, especially in that box. It's hard to tell from the picture, but you may not have enough bracing inside. My design calls for a front-to-back brace in the form of an "H" that is secured underneath the woofer hole and connects to the back of the cabinet. You may be spending a lot of LF energy wiggling the box.

What other items are in your system?