Pass XA 100.5 plugged into Power Plant Premier?

Mluding, undervoltage is not usually a problem in terms of reliability. The amp will run slightly less hot and maybe output 5% less power, not a big issue IMV.
Also, I would never use a Class D amp (PPP) to supply power to a fine class A amp. You would be adding class D distortion and HF noise to your fine class A amp.

Sorry, a typo - I meant the allowed deviation is +5% (% not Volts) and -10%.

Elberoth2, unfortunately the norm says that the allowed deviation can be +/- 10% (I think that's what you meant) and in fact sometimes the voltage also dips below the 200 V mark.

I guess the loss in power will not be a problem, but having "clean" power (THD reduction) also for the amps sounds intriguing and according to PS Audio the PPP should be able to handle also the power consumption of the Pass.

Nelson Pass, however, quoted "You can try it, but don't be surprised if it doesn't work out."

We'll see. I'll buy some new, longer, power cables and will just try it out

Electrical norm says that the voltage maximum deviation should not exceed +5V or -10V, so everything between 198V and 232V is still within the specs.

You will be fine running your Pass from 210V mains on 220V setting. It may be short of 5W of its maximum power, but that's it.

they will sound better straight off the wall