I built a house 7 years ago that has the identical span, and 12"OC. I don't like the feel of it to much either, but I don't know about this plywood idea. It would seem to add weight, and not support the joists in the direction they need it. Even 3/4 " plywood flexes. Blocking seems like a better plan, 2X10 10 1/2" pieces screwed to the joists and to the subfloor. Do your B'ment plans call for any partition walls under your F/room? Perpendicular to the joists, a wall would do the trick.