I am wondering if the, "O.P." ever had any luck. I have had the same problem with heavy speakers. Sorbothane "will" actually work. But you may want to contact the manufacturer directly for a baseline of recommended shapes, type/rating and etc.. "I have, several times". They were not difficult to work with. But their answers left me at times with just many more questions, especially upon further reflection.
I have used the (50/70) Durometer rated pads, (6" x 6") X 3pcs. at 1" thick. "Which brings the Natural Frequency down to about 10Hz. according to the manufacturer".
"Or so I was told"
I was also told that these particular pads were orig. designed for autobody to chassis isolation. Which made sense. They came with documentation and were rated at (1500-2500Lbs.) per pad. "When calculating for this type of application the objects weight plus the, (Inertial force of mass, AT load), and also force vectoring must be taken into consideration". Not just a speaker's given "dead weight". Otherwise, the Sorbothane will tend to simply disintegrate during it's very brief and tortured lifespan.
Anyway, I achieved what I'll call, "Mixed Results" at best. I ended up placing all of the pads behind bass transducers in sealed cabinets which did work well at reducing resonance and interior reflections.