Hi again and thanks to all who replied. I'll have to choose between
the "sister" joists attached lengthwise to the joists or the cross
braces between joists--it's helpful to know either of these will
strengthen the floor.

I appreciate those who worried I was cheated, but it's not the case.
The house is otherwise strong and sound with a secure foundation. For
most folks with normal furnishings it would be zero problem. My heavy
items are storage cabinets, bookcases, media racks, and related stuff
all in one large room or studio--that's the room that needs the
reinforcement.

I forgot to mention another method I used successfully in a smaller
upstairs room in another house, which was a layer of 3/4" plywood
screwed and glued to the subfloor under the carpet. That worked
really well. (Current house is single story, hence the jackposts.)
I asked my carpet contractor to do the same in this house in that one
room, but after the carpet was laid and all my furniture was in, I
discovered he had used 5/8" chipboard instead of good 3/4" ply. This
is softer and doesn't add enough rigidity. I could always empty the
room & try again, but maybe doing the joists will be simpler.

Art