Actually, the squeeks are due to the subfloor flexing. The tub was probably not supported very well. You can take some spray foam insulation and spray it up under the tub (below the base of the tub) through the access pannel. I have found this helps the tub to distribute the weight and either eliminates the squeeking or reduces it substantially. Use the hose type expanding foam that will allow you to get the foam as far back under the tub as you can.