How do I discharge the capacitors in my PS audio P20 ?

@frankmc195 If I had to wait 24 hours every time I needed to work on a tube amp...

Erik has the correct answer. Then after the wait ...

The nextpcb article above says to short caps with a screwdriver. I do not recommend subjecting power capacitors, especially electrolytic ones to a dead short as they may be permanently damaged.

It's possible that the PS Audio unit already discharges the caps on power down. Measure with DMM set to Vdc and relevant range. 

Assuming you don't have a 10watt 1k resistor on hand, you could use a 40w light bulb.