Hi Skrell,
Welcome to the Mattress Forum! 
I’m sorry you’re having a difficult time getting comfortable on your new Novosbed mattress. Unfortunately, it’s not possible for me to “diagnose” mattress comfort issues on a forum with any certainty because they can be very complex and there are too many unique unknowns, variables, and complexities involved that can affect how each person sleeps on a mattress in terms of “comfort”, firmness, and PPP or any “symptoms” they experience. There is more about the most common symptoms/pains that people may experience when they sleep on a mattress and the most likely (although not the only) reasons for 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, but there is no one firmness level that is “best for backs” in general because it would depend on the body type, pre-existing conditions, sleeping style, and individual preferences of the person. A mattress needs to have deeper layers that are “firm enough” to stop the heavier parts of your body from sinking down too far and make sure you sleep with your spine and joints in good alignment and upper layers that are thick and soft enough to “allow” any pressure points such as the hips and shoulders to sink down far enough to relieve pressure points in all your sleeping positions as well but “firm enough” and “soft enough” can vary widely from person to person.
As you are already on the plush model and then you attempted to make it softer without better results, this could point to a few things, such as potentially the need for a bit of a firmer surface comfort layer relative to what you’re correctly trying, or perhaps comfort layers that are of a different material and a bit more resilient (maybe latex versus memory foam), or perhaps a different style of support unit (springs or latex versus polyfoam). There really is no way for me to make accurate statements and the best I can do is provide this “theory at a distance”.
The other issue is that you already have a history of low back pain, and unfortunately there is no mattress guaranteed to cure that, as back issues can come from a variety of issues (but not limited to), such as flexibility levels, core strength, neural issues, disc issues, or a combination of all. Sleep ergonomic research points to focusing first upon alignment for addressing low back issues, then finding “just enough” surface comfort to work for your preferred sleeping style and preferences. There is more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel” that can also provide some useful insights into the reasons for pain and/or discomfort on a mattress.
The good news is that you chose a product with 120 night trail period, so if the mattress doesn’t work out well you can always return it for a refund.
In cases like this I always recommend another phone call to the manufacturer (and Novosbed does have a good customer service reputation, as you mentioned) and describe your situation and see if they have any other suggestions. They know their product the best and do want you to be comfortable, so I would contact them and see if they have any other ideas that they might be familiar with for people who have bene in a similar situation to yours.
I’ll look forward to learning about your progress.
Phoenix