Hi MarinaraBoss.
Welcome to our Mattress Forum. 
Sorry to hear that you are experiencing lower back pain on multiple mattresses, even after an adequate break in period.
To steal some words directly from Phoenix…
While there are too many unknowns, variables, and personal preferences involved for anyone to predict which mattress would be the best match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) based on specs or “theory at a distance” (see mattress firmness/comfort levels in post #2 here) … if your back issues are being caused or aggravated by your mattress (which seems to be somewhat likely because it goes away when you have been up for a while) the odds are high that you could find a mattress that will be a much better “match” for you.
The first place to start your research is the mattress shopping tutorial here which has all the basic information, steps, and guidelines that can help you with how to make the most suitable choice … and know how and why to avoid the worst ones.
There is also more about the most common “symptoms” that people may experience on a mattress in post #2 here that may help you identify why the mattresses you have chosen weren’t working well for you. The most common cause of lower back pain is an alignment issue from a mattress that either has a support core that is too soft or comfort layers (or toppers) that are too thick/soft for your body type or sleeping positions but a mattress that is too firm and doesn’t provide good support under the recessed parts of your body (such as the small of your back) can also lead to lower back pain.
The most important “requirement” for any mattress purchase regardless of any back issues is always to make sure that it provides good alignment in all your sleeping positions.
There is more about primary and secondary support and their relationship to pressure relief and firmness in post #4 here and in post #2 here that may help you visualize what good support and alignment “looks like”.
There is also more about the different ways to buy a mattress and how to minimize the risks involved in each of them if you aren’t confident that a particular mattress is a good match for you in terms of PPP in post #2 here that may also be helpful.
Would you also be willing to disclose your height/weight and sleeping position(s)? This is useful information for us in helping to determine where things could be going wrong.
Looking forward to your reply.
NikkiTMU