Sam,
You don't play around! The contact numbers that you received are excellent to have!!
The first thing I'd do is to accurately measure the Hydraulic System Pressure with a Calibrated Master Pressure Gage at the correct Idle RPM and adjust the Hydraulic System Pressure Relief Valve to 420.6 PSI.
I seriously believe that with the required System Pressure at Idle RPM all your flows will be where they belong. The reason I state that is that to achieve the advertised flows through Calibrated Orifices in GPM you need 420.6 PSI at Idle RPM. You DO NOT just raise the RPM!!! When you are running your Engine at any other speed above Idle RPM the Hydraulic System Pressure Relief Valve will maintain 420.6 PSI. I don't believe that the Hydraulic Flow of your Kubota Tractor is adjustable without changing the size of the Orifices.
As before, I don't have the Kubota B26 Manual but Hydraulics was one of my specialties of many and I've designed Hydraulic Systems, taught Hydraulics, and traveled Internationally as an Aviation Consultant throughout the Americas, Europe, Russia (Including throughout Siberia), Asia, and Africa. I was also an Independent FAA DME (Designated Mechanic Examiner) for both Airframe and Powerplant giving the A&P Oral and Practical Exams for many years before I retired.
Good luck to you,
Jim