Hi coffeesnob,
You can see some comments about Saatva in post #2 here and the posts and topics it links to.
I’m not sure you are clear about the difference between a box spring and a foundation. A box spring has springs inside it that flex under the mattress and there are very few one sided mattresses today that use them as a support system and in many cases they will invalidate a mattress warranty. Most mattresses in the industry today need a steel or wooden bedframe with a foundation that has minimal to no flex (vs a box spring that flexes) or a platform bed which also has little to no flex under the mattress. There are many in the industry that mix up the terminology between box springs and foundations even though they are very different products.
If you have a foundation (vs a box spring) then the manufacturer of a mattress will generally be the most reliable source of guidance about whether it would be suitable for the mattress you purchase (different manufacturers can have different warranty requirements for a support system under their mattresses) but if your foundation is suitable for your mattress and is still in relatively new condition and the support surface is still perfectly flat with no sagging and it’s structurally able to support your mattress and the people sleeping on it then it would probably be OK.
Phoenix