Historic house has red oak floors recently refinished except one room. The living room (1960's) addition has baby blue tile with white swirls. It has been removed to expose 3/4" ply on 1" X 6" planks 3/4" thick on joists 16" o/c. - Span 13' with crawl space underneath. Planks run perpendicular to joists. Tile is to be replaced with 3/4" t & g oak to match existing rooms.

The central issue is the existing sub flooring is exactly level with adjoining rooms finished wood floor. To place new oak on it would make room + 3/4" = trip hazard and would look strange as living room is connected to dinning room.
New oak floor will run perpendicular to joists.

Options as I see any others much appreciated.

1) Remove 3/4" ply and place 1/8" ply on existing planks and then 3/4" oak with lip (edge) sanded to match existing adjoining floor.

2) Remove 3/4" ply and bottom planks and place new 3/4" t & g ply on joists and then 3/4" oak flooring. This options will be flush with rest of house but is this strong enough relying only on the 3/4" ply>

Simply put overall goal is strength of floor and flushness with existing house.

Some friends have said 1X6s are desirable and should not be removed.

Thanks in advance

Ideally go down to the joists put in plywood (glued and screwed), less expensive, remove plywood, screw down the 1x's and lay the flooring on a 45 degree angle or take your chances running the flooring across the subfloor and with the joists.

The floor really needs to go perpendicular to the 1xs though, and if you rotate them to 45 degrees the 3/4 inch ply would now be stiffer.

If you use 3/4 inch red oak flooring the subfloor stiffness is not as important.

The oak is way stiffer than either 1x or 3/4 inch plywood.