If that trans has been set up for towing then someone probably installed a shift kit if the accumulator springs have been stiffened or some shims installed. If that is the case line pressure will be exactly right but accumulator pressure will be higher. Accumulator pressure cannot be measured. If it has no codes and everything electrical is working then if has to be a hydraulic issue. You need to pull the pan and get the accumulator body looked at. If you are doing it yourself be careful not to rip the vb gasket.
That sonnax valve is something I wouldn't consider. Ive built 100s of E4OD trannys and have never once had any problems with that velve having to be replaced. And by the way I work very closely with sonnax and there products on of there R&D shops is 10 minutes from me and I talk to greg or Bob Warnke very frequently.I am also a Tasc Fork member with Sonnax.