Has anyone tried a rubberized coating on older Klipsch tweeters?

I have used Dynamat on the outside of the horns in some of my Klipsch Heresy speakers.  I'm not sure I'd bother with the RF-3s.  I'd encourage you to move to the Heritage line or see if you can find some second hand Epic (CF-X) or Legend (KLF-XX) series speakers if you want better sound quality and like the Klipsch sound.  Bob Crites sells replacement tweeters for Klipsch speakers that are less fatiguing, but I don't think he carries anything for the "Reference" line.  

I've always wanted to try some Quartets out.  The RF speakers aren't bad by any means.  I still have a pair of RF-3 I in my home theater setup and also a pair of RF-5.  I've owned RF-7, RF-3 II, and RF-82 II also.