Hi bigchris,
[quote]Attributing the lower back tightness to possible excessive softness in the top layer, I asked James to set up an alternate configuration for me to try tomorrow: the base and core of the mattress is the same (3"XF / 6"MF), but instead of the 3"S comfort layer, I’m going to go with a 2"S comfort layer. Although I’m losing 1" of mattress, the overall feel should be somewhat firmer, thereby decreasing my hip compression, and curing my possible (or imagined!) spinal alignment problem. Yes?
This is worth trying, right? Or is moving from 11" to 12" going to be entirely placebo effect, unless I can try out both mattresses side-by-side? [/quote]
I think this is definitely worth trying and in theory it would do what you are suggesting (the pelvis would have less distance to “travel” before it was “stopped” which can certainly affect alignment).
I wouod take your time and trust your body and of course be aware of the possibility that your days experience can be an anomaly. I would tend to spend about 15 minutes or more after you are completely relaxed on a mattress in all your sleeping positions testing for PPP. I know that this can feel “odd” in a store but you will be sleeping on the mattress for a decade or more and it’s well worth doing what you can to make the best possible choice. Good manufacturers and retailers don’t mind a few return visits 
You can see some comments in post #9 and #10 here from earlier today about the effect that smaller changes in design can have … including a comfort layer that is a little thinner.
Phoenix