Hi 70sgirl,
Ideally you do the testing of a mattress as close to how you will have it as possible. An adjustable base, for instance, is a solid foundation. It may cause the mattress to behave firmer than if you test the mattress on a wood slat foundation which has a little bit of give to it.
Just in case you haven’t seen it, Phoenix has written an excellent tutorial on how to test to find the best bed for you.
https://forum.mattressunderground.com/t/how-to-look-for-and-find-the-best-mattress-for-you-read-first