Just an idea. If the dip is just between joists in a non-load bearing area and somewhere midspan try streching a string tightly across the subfloor in a spiderweb fashion about every 30 degrees with the center of the web at roughly the deepest point. Then use a little 1/4 in. masonite, a little 1/8 in. masonite, and a little 1/16 in. veneer and brad nail or staple pieces down to roughly fill in the pocket space up to the spiderweb. It doesn't have to be perfect if you are using 2 1/4 T&G, or 4, 6, 8, & 10 in. rough sawn boards. The problem, though, may be a cracked, split, termite infested, or warped joist or timber under the floor so some investigation would be prudent.