I intend to drive one Ref 250 with one P10. It should draw around 400 watts each channel and it should be fine.
In my experience, the P10 should not be used continuously over its 50% load. Some headroom is needed. It is particularly so with export 220V model. There is an extra step up transformer inside which add more heat to the inferno. It is a sealed box, there is no vent hole on its cover.
My two P10’s had to be sent back for repair after driving my original tube amps for 2.5 years to replace the output modules. Fortunately it was covered by warranty and it was free. Thanks to Paul for his help.
Now it has passed the three years warranty, I hesitate to drive them hard. I wonder it is advisable to place some washers under the top cover to leave a small gap for better ventilation.