What is under your TT, cheap islolation?

I would first make certain that the legs of your rack are sitting on top of your floor joists opposed to on the plywood between the joists. This will give you the most stability. You don't want to use anything that isn't rigid under the table. I use granite but it's not really inexpensive. You might want to try a large paver block 18" x 18" or something in that nature.

The bouncing is the result of vertical displacement from the movement of the floor. This will occur without regard to the type of isolation you use so long as the TT is supported on your floor unless your floor is braced to avoid flexing. Suspended TT's are especially vunerable to this type of movement, thats why folks recommend wall mounting. Interestingly unsuspended tables can often work reasonable well over such a floor. You could try using a large rigid platform under your stand insuring that the edges of the platform overlaped the floor joists. That might help but it might look quite ugly depending of the width of the joists and composition of the platform.

Most isolation devises are designed to deal with small vibrations. The vertical displacement is not really a 'vibration' that they are designed to deal with. If you can not eliminate the vertical displacement then the only thing that makes sense to me is to use something soft under the table like an air bladder based support.

FWIW

Not sure how cheap is what you consider as cheap but ...

As a turntable platform, I use two 24"x25"x1.5" maple counter tops laid flat on top of each other, separated by a cork sheet (1/16" I think), placed on Mapleshade brass cones.

It looks nice and really does wonders for isolation.

Regards
Paul

Look into suspending the turntable from the ceiling. That solves footfall problems and is effectively better than the 'self on the wall solution'.
Bob P.

Relating to the VPI Table, I'm wondering if you have the spring suspension on it, as you might be better off with the Sorbothane Pucks instead. Audiopoints replacing the VPI Rubber feet will help a bit, but these aren't cheap for a set of four. I bought some a couple of months ago for mine, and they seemed to help a bit, made the table look a lot nicer too, but one will get of sticker shock for the four 10-32 Points, and four coupling discs.

Another downside of the VPI, is its odd size footprint, making it harder to find readily available stone slabs, or wood bases.

An option I've thought of for myself as an inexpensive project, would be a DIY Sandbase (ala Brightstar) but I haven't done mine yet. Small 2'x4' Sheets of 3/4" MDF are available from places like Home Depot Home Supply, the Borders-Sides could be also made with 3/4" Oak, giving a nice look, and the Top Plinth of the Base could be sprayed with a nice texture-spatter paint to compliment the rest of the Base.

Even if you don't have a good saw, Places like Home Depot can do all the cuts needed for a very small fee, and all you'd have to do, is button everything up, fill the base with sand so that the Plinth sits level to the top of base, leaving 1/2" gap all the way around the base Plinth, and fill this void area with 1/2" Strips of Black Foam Weatherstripping for the finishing touch.

With all MDF, the cost probably wouldn't exceed $20 in parts. With 4" high Oak Sides, naturally will cost more. A base large enough for the VPI would no doubt be still suitable for many other turntables as well. Mark