Stuffing doesn't result in an appreciably reduced fb or f3, but it can lower Q. That can be effective in reducing a humped response in a too-small box. If you've got a response hump you can try stuffing and track the response to see if it helps. Measuring both near and far field in room is a good idea, as sometimes a hump is beneficial in countering room modes, boundary loading and cabin gain that add to response below the hump frequency.You can use REW to take response measurements of your sub near-field and certainly observe changes (as long as you measure exactly the same each time).
I know little to nothing about sub design, but I thought stuffing virtually altered box size, which should lower f3 (I would think). REW should be able to observe the change in the 3dB down point (again as long as you're careful about repeated measurements taken exactly the same each time).