FWIW, as per our plumber's advice, DH installed the first layer(s) of plywood subfloor before setting the bathtubs in our bathrooms, and then, laid the final layer(s) of plywood (so that layer runs up to the tub, not underneath it). After the tile floor was installed, the tub ended up being somewhat "sunken" into the floor. This subtle height elevation of the floor does seem to make stepping into/out of the tub a little easier.
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