Hi azcards4ever,
Welcome to the Mattress Forum! 
I’m sorry your current BME mattress seems to be a bit too soft for you. The good news is that you did choose a product that allows for an exchange/return should that become necessary.
Unfortunately your question doesn’t have any specific answers and there are a few “it depends” that are part of the answers that I might be able to provide you with, but I’ll do my best to see if I can help you figure things out.
First, there is information about the many different symptoms/pains that people may experience on a mattress and some of the most common causes behind them in post #2 here. Generally speaking, the most common reason for lower back pain is a mattress that has comfort layers that are too thick/soft or a support core that is too soft.
Starting with your BME, it is recommended that this be placed on a surface that is flat and firm. The foundation from your Saatva is a basic wood platform construction, so that generally would suffice. Your bedframe should also be fine, and you’d want to make sure that the center support for that is in place. So I’m assuming that the frame/foundation set up in your master bedroom is appropriate for the BME mattress and gives a more accurate representation of how the mattress is supposed to perform.
Moving to the guest bedroom, you describe the old bottom part of your mattress set as a “box spring”. Is it actually a true coil box spring that has quit a bit of flex to it? This is not too common, even going back a few decades ago. Is it a folded torsion system that flexes a bit but not as much as a coil box spring? And does the frame in that room have a proper center reinforcement?
The reason I’m curious about the actual construction of the guest room “box spring” is that it would provide some insight into what you’re experiencing. Assuming that it is an actual coil box spring, it might be allowing your BME to sink in more than when placed upon a proper flat surface, allowing you to sink in more deeply, which could point toward you having a “learned alignment” where you prefer to have your hips sink in more deeply with an accentuated lateral curvature. As you’ve only had your current mattress for two weeks, it might not be enough time for your body to adjust to a better alignment. An “active component” under your mattress generally impacts your deep support characteristics, and not so much your perception of surface comfort. It could be that you respond best to having something under your mattress that does allow for some “give”.
Another difference is dynamic of who is sleeping in the mattress. I would test out both of you sleeping on the BME in the guest room and see if you still sleep well, as having two people in the mattress also can impact your alignment.
Besides those two thoughts and the link I provided above, these are just my best guesses and “theory at a distance” as to the “why” of what is happening, and without more specific information I wouldn’t have a much better idea than those thoughts.
I’m curious to learn the actual configuration of your “box spring”.
Phoenix