WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Indeed you should be able to use the PY500,s. They are reputed to be more reliable in the long term.
However you have to face the fact that you will need two of them, and if my memory serves me correctly - they draw considerable heater current - which might mean a seperate heater transformer. I seem to remember that underrunning them might adversely effect their durability.