There is no true subfloor, there is an existing tounge and groove 2 3/4"
hardwood strip floor nailed directly to the joists and joist extenders. this
floor is a little thicker than 3/4" and has been carpeted most of the time.
the floor has been carpeted for 50 years i imagine.

I had a similar situation at the other end of Chesapeake Bay from you.
I found a flooring firm who did lots of old houses. One section of
house they floated a floor on top without problems, the other section
they said "do you realize how much this old floor is worth?!?!?!"

Before you do anything irreversible ask a dozen of your rowhouse
neighbors who did their floors. Bet there are two or three firms
commonly used.

In my situation I carefully pulled up the old tongue and grove, a
regular carpenter repaired the subfloor, and the flooring guys then
deployed.

Good luck! Your friends in new houses in the suburbs will have
sagging floors sooner than you will.