I have 1 3/4 inch redwood t&g subfloor, and am installing slate tile. I determined that I have a good deflection rating, by using the deflection calculator at johnbridge.com. Can I use CBU directly over the subfloor without first laying down plywood? Should/could I use Ditra in addition to the CBU? I need to keep the underlayment+tile to 3/4 inch for a smooth transition to floor in the next room.

Personally, I can not see any upside to laying down plywood over an already excellent subfloor. The Ditra sounds like an excellent idea, although I personally don't see any issue with putting the cement backerboard directly on the redwood.

I'm curious to see what the guys on here that do this for a living think.

Actually, there's a very GOOD reason to add the plywood. You DO already have all the strength you need. Unfortunately, you don't have the stability. Dimensional lumber expands and contracts with the seasons way too much, and as a result, you CAN NOT lay CBU directly over dimensional lumber. All you need is a 3/8" layer of plywood, and you're good to go.

Thanks for your reply, Bill. I live in California on the coast, where seasonal temperatures are relatively mild - it rarely gets above 70 or below freezing. I was told that I could lay felt over the redwood, followed by 1/2 inch Hardie, as the floor is only 30 sq ft. 40 yr contractor specializing in custom homes gave me this advice... bad idea even with mild conditions?