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

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. 

You culd also just unplug it and wait 24 hours

Ask PS Audio 

Erik has the correct answer. Then after the wait place a pair of pliers across the two terminals of the capacitor to make sure it has discharged completely.

Power down. Unplug from the wall, turn power switch back on. This should drain any residual charge left on any power supply caps.