Hi PeterDoubt,
There are a lot of people (consumers and in the industry) that use “box spring” and “foundation” interchangeably when they are actually different products. A box spring has springs inside it that flex under pressure (such as pressing with firm pressure with your hands or knees) and a foundation doesn’t have any springs and would have very limited or no flex. A foam mattress generally does best with a foundation that provides non flexing and even support under the mattress. If you put a foam mattress on a box spring or any support system that flexes then it will change the feel and performance of the mattress.
If you have a box spring that flexes then you can approximate what your mattress will feel like on a foundation (or a platform bed that also has a non flexing slat or solid surface which is suitable for directly supporting a mattress) by putting it on the floor or by putting a sheet of plywood over the box spring so that there is no flex under your mattress.
There is also more about the different types of support systems that are generally most suitable for different types of mattress and some sources for each of them in the foundation post here.
Phoenix