I use mason industry spring isolation mounts (3" deflection). I looked around the web for a mass-spring calc, but wasn't able to find one given the variables. Math was to complicated for me...

-but- I knew I was underloading them to start (based on their rating) and I found that playing LPF'd pink noise I was able to find the resonance going through the floor (wood joists) and into a mic in the tracking room.

I just used some sandbags then to weigh down the speakers (which are big and weigh 180lbs on their own) until the resonance was below 10hz. My speakers don't have much output down there so it was harder to test. Plus I can't hear it and don't know how much the mic dropped off too...

It was interesting because you could hear that the decoupling was worse at the resonance frequency when I started (it was around 35hz to start).

After doing that, I'm kinda mind boggled that any mass-spring isolator isn't more specific as to the mass variable. If you underload it, it makes it worse.