The sandboxes I put under my speakers made a huge difference from just being on the suspended trampoline-floor. The bass went from "one-note" to playing tunes and the mids and treble became clearer, as well. As soon as it stops raining I'll be bringing home 6 of those 4x8 panels that are white finished over stiff fiberglass, like a giant ceiling tile. Very good dampening I'm told. $11 apiece, worth trying.