I'm assuming you have a fiberglass tub/shower unit. The squishy feeling you describe is the tub bottom flexing. Before setting a fiberglass tub, a good plumber will take the time to place mortar or plaster of paris underneath the tub to give the bottom some support. have been with him for 5 years and have never put anything under the tub and never had 1 complaint. Lasco and sterling both say you dont need anything under them. I dont know about National, kohler, or Aker.