What is the finish flooring?

A large part of the flooring will be ½ or ¾ stone tile, part ceramic / some ½ porcelain tile, balance ¾ hardwood. We want to achieve a barrier & threshold-free floor t-out -- no visible or discernable changes in floor elevations, and reduce exterior door thresholds.

Oh, and staple underfloor radiant heat.

--Mark

A 1 1/4" subfloor will be overkill for hardwood, just adequate for stone. (I'm assuming at least 2x10 joists @ 16" o.c.)

But it will be necessary for what you're trying to achieve.

Instead of doubling up on plywood, I've used Ditra decoupling underlayment. To reduce squeaking floors, I glue, nail and screw down all subfloors. I-beams instead of dimensional joists would also help.

I went to great efforts on one place to ensure the various flooring surfaces were level. When I sold it, I was so proud of this detail. Buyers could care less, so I don't do it anymore.