The subfloor is about 1.5" thick. This is how thick the subfloor is on
the cover to the craw space (on top of the 1.5" is the actual hardwood
floor on the cover).

I didn't know the subfloor were not the original ones when I started
the job, had I know that, I would've insisted on either 1/2 hardyback
or a layer of plywood.

No need for the additional ply- what you have is definetly 2x6 if it's
really 1.5inch. Good solid floor. Don't worry about it- it'll
outlast you!