Hi ByronSleeps.
Welcome to our Mattress Forum. 
I’m sorry you’re experiencing lumbar pain on your Alexander Signature (now renamed the Sparrow Signature Hybrid). Ongoing lower back pain is quite frustrating and is an indicator of any number of things, most often improper spinal alignment from incorrect mattress support.
In other words, lower back discomfort can be caused by a mattresses that are either “too firm” or “too soft” on the surface (common causes of pain when sleeping upon a mattress are listed in post #2 here ). If the mattress is too soft on top or not supportive enough, it could allow you to sink in too deeply and allow for excessive sideways bending of our lumbar/thoracic region. If the mattress is too firm, it may not allow your hips to sink in quite enough and this could leave less support for your lumbar region, which can then allow an excessive lateral bend in this area.
Because you went from medium to firm and are still experiencing pain, my guess is that perhaps you’re needing a softer foam as opposed to a firm foam. Would you be able to get your hands on a soft 2" topper that you could use to test whether or not this helps alleviate you lower back pain?
NikkiTMU