Interesting. Now this may be a total red herring but I had a Mk1 TX (although I doubt the MK is relevant) that was running around 9. No matter what I did, new spring, new breech seals, new piston seal, increasing pre-load ... anything, I could not get it to increase.
Then I resealed the end plug on the compression tube. Perfect ... full power resumed, and adjustable. The O ring is 1.6mm x 22.1 but the thread needs more sealing support from the likes of a suitable Loctite. I have heard of the PTFE plumber's tape being used.
More on the double breech seal: further investigation, going back to 2018 suggests a chap called Bigtoe used to make them. That's all I know.
Cheers, Phil