I had similar problems with my 8000Fs (I use a pair for "wides" and for "rear surround." I did to the 8000Fs what I do to every Klipsch speaker I've owned (except the Palladium P-39Fs, too difficult to disassemble and standing waves in cabinet mitigated by "boat hull shape). I took them apart, improved bracing (L shaped wooden dowels across the cabinet - thrice longways - and 1x2" wooden dowel along each panel side). I re-glued with 2-part epoxy (5min JB weld epoxy flows well) and then another layer with super glue. I lined the ENTIRE inner surfaces with TWO layers of Dynamat, followed by ENTIRE surfaces with No-Rez, followed by polyfill stuffing. Each driver housing was also damped with dynamat. Upgraded cross-overs from GR Research (THIS IS IMPORTANT) and then soldered connections throughout. They sound amazing. (PS - I did the above to my RP-600Ms, RB-75s, RB-81s, 8000Fs, RF-7IIIs, and SPL-150s - improvements across the board with resonance and subs were able to dig 1-2hz lower).
