Andrew Barss wrote:

I'm hoing to locate a local dealer Delta-FL, which looks like an
excellent product for this (it's a heavy, dimpled polystyrene
subfloor).

You may want to check and see if there is a Lumber Liquidators (
http://lumberliquidators.com ) store near you. Someone here mentioned that
to me recently and I was amazed at how much better their selection of
products and prices were that Home Depot, Lowes, etc. And, some of the
specials sale items they have are way less expensive than any other place I
have seen.

I agree with the person who suggested taking up the existing tiles if it's
not too much trouble, placing them in plastic bags, and throwing them out.
Almost everywhere it is permitted for homeowners to do that with those types
of tiles even if they do have asbestos in them. If it is concrete
underneath, it should not be too much trouble scraping the up and trashing
them. I think I read somewhere that if the tiles are 9x9, they may be from
a time when some of them had asbestos, but if they are 12x12, they almost
certainly do not have asbestos. But, either way, I'd say just remove them.

Then do floor leveler, maybe just to even out the low spots, then plywood
over top.