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

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.

bmbmzig

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

No. That will recharge the capacitors.

Cleeds, et. al. - 

I don't know if there is any standby power battery in the described device, but if the unit has been unplugged from the wall, ANDS REMAINS UNPLUGGED, I am not sure where the power is coming from to recharge the capacitors when the power button on the device is pressed. Press and hold for a few seconds may be worthwhile. 

This procedure is standard practice to ensure that a computer system's PSU and other components are fully discharged before pulling circuit cards, drives, etc.  Also, since the user may not know the polarity of the electrolytics in question, they should watch carefully for ANY change in readings on a test multimeter - either up or down scale - which would indicate that some residual charge remains in that part of the system.

Finally, testing should be done on a DC range similar to the rated voltage indicated on the side of the capacitor being tested.  More, and you may not notice any "twitch" of the needle, and less may result in damaging the test instrument or device being tested.  Also, given the internal workings of the test device and the voltages involved, there may be only a momentary indication of residual charge, not a steady-state reading.  

@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 ...