The tub manufacturers recommend that the tub be set in mortar or dry wall mud to help deaden the sound and for better support.
It does not sound like it was done.
We have had some luck by using long flexible hoses you can find at an automotive store, attached to a can of foam insulation, spraying it under the tub where needed.
Of course a small hole/s needs tobe drilled near the tub apron or front or back in a area that will be easy to conceal after the job is done.
I look for baseboard I can remove or a closet that the tub wall is on so access is not noticeable when done.
» This message has been edited by PLUMBILL on 02 April 2006