Hi tropicana,
You certainly purchased a good quality/value mattress but it can be risky to buy a firmer mattress with the intention of adding a topper when you can’t test the mattress/topper combination in person and compare it to other combinations because choosing a suitable topper that is a good combination for both you and the mattress you are using it on (different mattresses will affect the choice of topper that works best for a particular person) can be almost as challenging as choosing a mattress that doesn’t need a topper in the first place and you have more variables that you need to “get right” to find a sleeping system that works well for you “as a whole”. The only way to know for certain whether any specific combination will be a good “match” for you will be based on your own personal experience so you may need to go through some trial and error to find a mattress/topper combination that works well for you (see post #2 here).
It’s very unlikely that the quantum coils are the issue and it’s much more likely that the comfort layers that are already in your mattress in combination with your topper just isn’t a suitable combination for you.
While it’s not possible to “diagnose” mattress comfort issues on a forum with any certainty because there are too many unique unknowns and variables involved that can affect how each person sleeps on a mattress in terms of PPP or any “symptoms” they experience … there is more about the most common symptoms 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.
There is also 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”.
These posts are the “tools” that can help with the analysis, detective work, or trial and error that may be necessary to help you learn your body’s language and “translate” what your body is trying to tell you so you can identify the types of changes that have the best chance of reducing or eliminating any “symptoms” you are experiencing (at least to the degree that any symptoms are from your mattress rather than the result of any pre-existing issues you may have that aren’t connected to a mattress).
The most common cause for lower back pain is a mattress that is too soft (either a support core that is too soft or comfort layers that are too thick or soft) and if I had to guess I would guess that with your topper you have probably added too much additional thickness/softness to your mattress.
There is more information about choosing a topper in post #2 here and the topper guidelines it links to that can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success.
Phoenix