I've had good luck with these- Sorbothane ovals from Mcmaster-Carr.com
"Selector Pack— Includes five plain-back 1 1/2" x 3" x 5/32" thick oval pieces, one of each durometer (30 OO, 40 OO, 50 OO, 60 OO, and 70 OO). 8514K2 Per Pkg. $11.45"
Get 2 of the packs, one per speaker and use all 5 ovals per side. Maybe this will help to disperse resonances, by using all different durometers?
No guarantee they won't mar anything, but I've read that you can put plastic wrap over them so they don't touch the speaker.
If you hate 'em , you're out $25, not too bad in this hobby.
-Mike