Hi lupo,
First of all I should mention that it’s important to make sure you have slept on the mattress for long enough to make sure that you have gone through any adjustment period that may be necessary for a new mattress (particularly if it is much different from what you are used to) or at least that it is obvious that your “symptoms” are consistent and there is little chance that they will improve with time before you consider your other options.
I would also make sure that your mattress is on a firm non flexing foundation to rule that out as a possible cause.
Lower back issues when you wake up are generally an indication of either support layers that are too soft (unlikely in the Beautiful) or comfort layers that are too thick and soft for the alignment needs of the person sleeping on the mattress.
Unfortunately … there is little that can be done in these cases and the best solution is generally an exchange for a mattress that has thinner or firmer comfort layers if this is possible. the reason for this is that even with a firmer topper … it will generally still “bend” into the softer layers below it.
It may be possible to add say a wool or other natural fiber mattress pad or topper or even a firmer latex layer on top which may help prevent your heavier pelvis area from sinking in as far but this will change the feel and possibly the pressure relieving qualities of the mattress and there is no way to predict success for this with any particular individual.
If you do go in this direction rather than a mattress exchange … I would focus on sources where the topper you use is returnable in case you are unsuccessful. For example this silk topper from Costco or one of the wool toppers listed in post #3 here may have some effect.
It’s always much easier and more effective to soften up the comfort of a mattress that is too firm than trying to improve the support of a mattress that is too soft.
Phoenix