Should we remove multiple layers of subfloor before installing?

Q imported from our old site, Face Lift Floors: After removing carpet from my bedroom I discovered that there are 2 subfloors, one on top of another.

I want to install hardwood floors and the flooring company suggested we put the 3/4 inch hardwood over both subfloors. If we do that it would create a level difference from the hallway into our bedroom as well as a difference in the threshold from our bedroom into the bath.

It also crowds the return vents in the wall. The flooring company suggested we use risers for the floor difference and cut the return vents down an inch. The risers would work from the hallway to the bedroom but probably not with the marble threshold leading into the bathroom. Also, the thought of cutting my vents and cutting the hardwood to work around the case molding seems sloppy to me.

Should we take up the top subfloor which was secured with screws, nails and liquid nails? (Maybe more trouble than it is worth?) Or put the hardwood down over the 2 subfloors?

A: Since removing the carpet, what was it sitting on directly? What do you see when you walk in the room? Plywood? Tile? How thick are the 2 subfloors you mention and what is installed on top of them?

I am not a fan of multiple layers. Every house is built with some sort of subfloor, usually 3/4 thick. Pine planks installed diagonally. Pine tongue and groove, installed front to back. Plywood. Chipboard. Depending on the condition and stability of the original subfloor, I may or may not recommend installing at least 3/8 plywood, screwed into the joists, and then installing the finished floor on top. I think if you start to build up multiple layers, it is time to strip it all down. This is the best way to ensure a stable floor in my opinion.

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