I pursued this thing after reading an old Bee Pre post of Doc's.He described the use of a pneumatic inner tube as the base of a plinth for reducing microphonics in tube gear. This is a ROHO Cushion. These are used by individuals who are wheel chair bound and the cushion helps to reduced the occurrence of pressure injuries caused by spending many hours seated in a wheel chair. I found this one on Amazon. It was sold as used but actually had never been used, it was in a factory sealed box.I paid $125.00. The prices are all over the map so careful shopping pays off.
On top I place two pieces of limestone which have a combined weight of about 50 lbs. These cushions are designed to handle the weight of an adult man or woman so you could probably go higher than 50 lbs., but I just happened to have this stone left over from a counter top project so I used it.Also, I have no idea how much air pressure is in the cushion, I just pump it up until the sag is out of the cushion. I'll have to play with this to see if there is some optimal pressure, but I will need to come up with some way to measure it as the valve does not have a stem like a tire does, so a conventional pressure gauge will not work with it.(BTW, mine came with it's own pump so if you pursue this, make sure you get the pump with yours).
So far I have put several different components on it (including a Quickie) and the reduction in microphonics due to footfalls, etc. has been pretty impressive. I really haven't tried to fine tune anything, I just pumped it up, put the stone on it, pumped it to take the sag out and thats it. For scale, thats a Crack on it. I think I'll just build a wood base around it and call it good.
PS this one is a High Profile Cushion. They come in different heights and sizes but I figured this one would be an all around useful size.
Jamie
Here's some images: