thin-set

I am about to set an Americast tub - Cambridge model - and am deciding if I should bed the tub in thinset or minimal expanding foam. American Standard says that it isn't necessary but it sounds a bit hollow to me when standing in it. Any suggestions are welcome. Thanks.

Hi everybody! I know, I should ask it at johnbridge.com forum but they don't register new members for some reason... Anyway. I'm moving slowly on my first bathroom project :) And new questions are popping out. They sound silly for professionals but I'm newbie :) The question: what thin-set...