Hi thatdubstepsound and thanks for your post. Sorry to hear you have recurrent issues with your low back area. Reading between the lines I think we can summarise as follows.

1. The initial injury was due to deadlifting 11 yrs ago. So this may or may not have been muscular or ligamentous in origin. Either way it may have cause some persistent instability in the region and XRays will not show these up. Diagnostic Ultrasounds also not perhaps that useful in this region especially if they were not dynamic scans i.e. scanning while you were moving.
2. It appears the discomfort is positional if lying with a roll under the low back (giving you some lumbar extension) and knees flexed (reduces perhaps any pull on the lumbar spine from the psoas muscle (hip flexor)
3. Overhead pressing is an aggravating factor (many movements in the lumbar area in this activity) suffice to say that the instability makes this movement problematic.
4. Moving from a bent over position, even with body-weight, and returning to an upright position is now causing issue.

The first thing to know is that PAIN is just the brain telling the conscious you that it is not happy with what you are doing. It is not related to actual tissue damage or inflammation. It is the response to signals from that area sent up to the brain saying 'Please deal with this'. If you satisfy the brain you made a change OR if you reduce the stimulus then the PAIN reduces or resolves until you again put the tissues/structures under the same 'duress'. So most commonly there are mechanical and not inflammatory reasons behind it all. And in your case it is possible that the initial injury causes some deep structure to become injured and they are not been able to repair to a point of providing the original structural stability in that location. The result of this is that other muscles and their tendons have to try to compensate and take over more of a stabilising role and once you bend forwards toward 90 degrees these become of little to no help at all. That's just the way the body is designed (it protects some structures under load in forward flexion but then cannot support those same structures under that load).

I suppose I am writing this as if I was thinking out loud and it is saying to me that perhaps you have a chronic underlying instability at a deep level (lumbar spine/ligament/even a floating rib connection). One that is perhaps always going to be there. At times certain movements cause either spasm in the supporting structures or that the support itself at times pulls the unstable area into a less than desired position triggering signals up to the brain and the brain says PAIN!!

And so what to do? Firstly you need a very hands on therapist who can work with you to finding the root cause/region, and even stimulate the aggravating movements to specifically see what is leading the chain of events to the alert signals being generated. It goes without saying that I would think about changing any gym type activity you have done for many years to a new approach. I think stopping all exercise when you are someone who likes to exercise is never a good idea. It might be that swimming, gyrotonic (perhaps especially useful for you), pilates reformer work, some forms of yoga will help you to move in a slightly unloaded fashion where you can explore those movements and directions that are the aggravating ones. And I think that is key, the therapists need to help you to depend your understanding which is not something a scan can do. Once identified you can work together on exercises that feel supportive to the movement, gradually increasing load and difficulty over time. But start small and little load as you need to get the larger muscles to stitch off in order to get the deeper structures to fire up and get working.

I hope you find some of my comments useful, even if a variation on the way you are coming at the problem. It is an issue that is totally normal within the population. 85% of working age people have back pain at some stage albeit at different times and ages.

Let us know any further insights you have along the way as it would be interesting to contribute some more.

regards