What to do? Jack the heezy!
Working with a pair of large house jacks it looks like our man squoze an inch out.
However, he now tells me he thinks he could get another inch out with two larger jacks and thereby eliminate the need to do much leveling of the main floor before laying down the hardwood floors (and by "leveling" I mean essentially filling the large dip with a layered plywood puddle to create a level surface). I'm all for anything that eliminates the need for adding material to compensate for sag since it just adds more weight and more work and is bound to have side-effects.
Check-ch-check-ch-check-ch-check it out! (click on pix to enlarge)