Ricardo,
I have a very similar problem with my HT L/R speakers. I ope you don't mind my asking Pete a question...
"One other point about stands changing the sound: besides diffraction signatures changing slightly, the stand itself can be a contributor to the sound. It too needs to be very solid, inert, in order to counteract the acceleration forces of the driver."
Pete,
My HT speakers are set atop cabinets that resonate like crazy. (My wife likes them!) Can I use some sort of rubber pads on the cabinets; add a block of wood or stone (or??) on the rubber; place rubber pads on the block, and my speakers on the pads? Will this eliminate most or all of the resonance of the cabinets?
-Michael