I had good results with damping the back side of the piezo with mortite. I don't know if this will do what you are wanting though. There was also talk of disassembling the tweeter and using dammar varnish on the diaphram. If I recall correctly, this was from the single driver website where the Motorola piezo's get a decent amount of use. I haven't tried this and probably won't. The biggest difference in sound came when I switched from a pretty good quality solid state amp to a better push pull pentode amp. Better bass, midrange, and highs. Really transformed this speaker. I think the sound could be characterized as a warm, full bodied sound with hifi characteristics, yet possesing drive and dynamics not typically found in "hifi" speakers. In a nutshell: great sound with plenty of balls!
Good luck with your speakers and keep us posted. Jerrod