We had a tweak demonstration for our Audio Society this weekend, and I would have to say that the innertubes under all components made a noticable improvement in all of the demos, both for soundstaging and for the leading edge of transients. You have to play with the air pressure--it shouldn't be too high, or you start losing the benefits. One thing our demonstration also tried, which many of us liked, was the use of silicon in the tubes. Tom, Redkiwi, you might want to try something like that as well as oil (not car oil, you're right, it would degrade the rubber), which we also found to be beneficial.
Turntable innertubes: air, water, oil
My turntable sits on three 4" squares of Navcom rubber .75" high, on a Torylite board that rests on two 12.5" x 2.25" bicycle inner tubes placed within a shallow wood and medite box. At first the tubes were partially inflated with air, then I removed the valve cores and filled them at the kitchen sink with 16 ounces of water each. I put the valve core back in and the cap on, and have had no problem with leaking. Going from air to water clearly improved the sound. In the past few days, I've replaced water with 15 ounces of mineral oil, all I could get in. Mineral oil is thicker than baby oil, and while I wanted a thick oil, I was afraid car oil or transmission fluid would attack rubber and cause a spill too horrible to contemplate. The challenge of getting oil into innertubes can be met thus: in the middle of a 24" by 1" piece of wood, drill a hole just big enough to take the valve stem, then drill a second hole meeting this one at a right angle, through the .75" thickness of the wood, just big enough for a .25" bolt to be threaded into, cutting threads in the wood as it goes. Pass the valve stem through the first hole and use a .25" bolt in the second hole to clamp it firmly in place. Rest the ends of the 24" stick over a large pot or bucket, to catch spilled oil. I used a squeeze bottle with a pointed nozzle for the filling. It took me more than an hour per tube. For most of the filling, it was hardly faster than a drop at a time. I'm sure there must be a better way to do this, but I'll leave it for others to find. One problem is that air must come out as oil goes in, even if the tube is squeezed flat at the outset. The sonic results were worth it! In a word, the sound was less blurred, in sharper focus. Details that had been vague before, like quiet jazz cymbal work and accompanying bass, were now vivid. Instruments sounded more "present", and interplay between instruments was striking. Dynamics were sharper. I think that oil improved more over water than water improved over air, but so many changes have occurred in my system since I used air that I can't be sure. I wonder if any commercial suspension device uses anything but air. If not, there's room for a major improvement here, IMHO.