AKA made low pressure / high flow valves for Spyders and Cockers. Kmac has some pictures: Autococker Valves & Valve Kits Photo Gallery by Kenneth MacDowell at pbase.com
The main design changes were the larger valve face and the flower shaped cup seal. Bob long made a similar valve at one point with a brass cup seal almost the same shape and a ramped output. There aren't that many of those out there so I suspect there may have been some sort of patent issue.
So one thing to think about is the size of the valve face. Not sure what would be optimal but it's been pretty well proven that a larger valve face allows for lower pressure. Actually, a large valve face may require
lower pressure as the high pressure gas pushes the valve closed more quickly.
I realize most of this relates to sheridan / cocker valve setups and gas flowing through a powertube is a bit different than out the side of the valve but I think that some of the designs might have some application here.