is it necessary to pour concrete under the new tub or is flat earth ok?

Hello and thank you for your question. You should have a “firm level surface.” If you prefer not to use concrete, other options are well laid pavers, or this portable plastic pad system. It has been around for years and is a nice alternative to having to mix/pour concrete. http://www.spasandstuff.com/spa-accessories/misc/handi-spa-pad.html