Sagging floor reinforcing
We've got a house built in the early '70's, with all of the usual weird early '70's construction experiments. The floor joists are 4x6's on 48" centers with a 1.25" plywood subfloor. Strangely, with a span that great, the ply is starting to sag between the joists (the joists themselves are fine). Obviously, I need to do something to reinforce it. Construction isn't my area of expertise, so that brings me here.
From everything I've read, it looks like 16" centers is the current norm for framing a floor. I'm wondering if it would work to use joist hangers and 2x4's to span between the joists on 16" centers to reinforce the floor. A friend, who's not a professional carpenter, suggested that using 2x6's might be better, but I've also got to install a radiant heat system between the joists, and in order to run the pipe for that, I'd have to drill a line of holes through each of the 2x6's (which I imagine would compromise the structural integrity, or at least render them as efficient as a 2x4). If I can use 2x4's, then I can run the pex under them, and still have pipe run recessed between the bigger joists.
Are 2x4's on 16" centers structurally acceptable for framing between the existing 4x6 joists to reinforce this floor? Are there going to be problems with that?
I expect that I'll have to either jack the plywood up (at most it's sagging .125" or so, but there's a considerable amount of "bounce" if you hit things just right) or figure out some way of lifting it from above. It's possible that simply pulling the nails on one edge of the ply, adding the reinforcement pieces, then reattaching the ply might work.
Thanks in advance.