Here's what I have now: 15.5" ceramic tiles laid badly by previous owner (a few dabs of construction adhesive under each one, needless to say they're all cracked and coming up), on top of 1/4" or possibly 3/8" plywood (now all damaged and coming up due to damaged tiles), on top of subfloor of diagonal 1"x8" boards on the joists.
Now, I'm going to have to pull up the tiles and that nasty plywood, but then should I just smooth out those diagonal 1x8 boards (and what would be the best way to do that?) or should I put down 1/2" or 3/4" plywood between those and the underlayment (it's in the kitchen, I'm considering vinyl flooring for now, and keeping in mind more expensive options in a few years time).
Something to think about is that I don't have much height in the cabinets' toekick. It's about 2 1/8" above the tile surface, which makes it about 3" above those diagonal floor boards.
For tile, you need a minimum of 1 1/4" of rigid subfloor, with the top layer being a tile backer board(duroc or hardibacker). Diagonal 1x6 subfloors generall have a bit of bounce to them. I would add 3/4 plywood screwed to the joists, and a layer of tile backer glued to the plywood.