Re: homeowner question
You'll need 4x6s, 2x6s, plywood (same size as existing), and joist hangers. Plan on removing the tub since you say that the tub keeps sinking. You'll probably need some bottle jacks to hold up the subfloor while making repairs. You won't need the crawl space since you'll be pulling up the subfloor to make repairs to the damaged area. Before closing it back up, make sure to apply copper green to everything you can reach (beems, girders, plywood).
YOU WILL BE OPENING UP A CAN OF WORMS! So be prepaired. Good luck.