Larry:
Yes, "the pressure comes from the pump port in the loader valve and is fed through the PB port on the loader valve back to the BH90 and is returned back via the PB port on the hyd block".
Relief valve: when valves are in series (as they must be on the typical small tractor open center system) the pressure at any point in the circuit is limited to the lowest setting on the upstream relief valves. Therefore, the 2000 psi relief valve in your new aux valve, which is upstream of the backhoe, will stop the pressure buildup in your backhoe at 2000 psi (actually a little less than that because of friction losses in the valves and lines between the aux valve and the backhoe). That may be enough pressure for your backhoe, but I don't know. Suppose your backhoe is designed for up to 2500 psi and that the system relief on your tractor is also set for 2500 psi. Before you install the aux valve the full 2500 psi is available to the backhoe (assuming no relief valve in the loader valve, or if there is one that it is set at 2500 psi also, or higher), but once you install the aux valve the max pressure at the backhoe will be limited to the aux valve relief setting of 2000 psi.
Unless it will overload an implement attached to the aux valve, you can increase the loader valve relief to 2500 psi (in my example) to get full pressure to the backhoe.
As a further complication, if the system relief is set at, say 2800 psi, and you want to get a full 2500 psi to the backhoe while its cylinders are moving (as opposed to being stalled against a load), you would set the aux valve relief at 2550 psi to offset the losses in the circuit from the aux valve to the backhoe (50psi is probably high in your case, but it is a rule of thumb).