[Wavery]

I'm not sure why I am just now seeing this but if you haven't attacked the issue yet, here's an EZ fix.

I recently removed the outside shower box (for a different project) and reviewed the under-tub area. I remembered that our tub had a soft spot where yours is. I noticed that there was a gap between the tub bottom and the floor. I got a can of that expanding foam and filled that area. I left it set for a few days, then I trimmed the excess foam so that I could get the outside box back in. I tested the tub and it's solid as a rock.

Hope that helps.