Hi Dt3,
Thanks for the update … and assuming that you decide to pull the trigger … congratulations on your new mattress 
It would depend on whether it’s an actual box spring or a foundation. A box spring has springs inside it that flex under the mattress and a foundation has has no springs and has little to no flex. Box springs are uncommon in the industry today because most mattresses are one sided and need a foundation rather than a box spring. A box spring is most commonly used with two sided mattresses and can play a noticeable role in the comfort and support of a mattress so if a two sided mattress comes with a box spring and that’s what you tested it on that’s what I would use.
There is more information about different types of support systems (including bedframes, foundations, and platform beds) that are generally the most suitable for different types of mattresses and some examples of each of them in post #1 here.
My first choice would always be to use the foundation or box spring that is recommended by a knowlegeable retailer or manufacturer and it’s always a good idea to check with the manufacturer of your mattress to make sure that any support system you use is suitable for your mattress and meets their warranty criteria.
Phoenix