Is your Pass X250 dead silent or slightly buzzing?

Try a PS Audio Humbuster. You might have DC potential riding on the AC. I had the same thing with my Pass-designed First Watt amp, as well as other amps I have used. A small problem that nagged at me. PS Audio has a 30 day money back policy so, why not give it a shot. It took care of my mechanical xfmr hum. Ahhhhh!

Yes, i can hear it probably due to 90dB efficient speakers at idle but when the sweet tunes start it's gone. It's even worse now w/tube pre but not audible when playing. one of those nagging things i've put on the back of a long list. enjoy your x250.

It's not related to Class A circuitry, that much I can tell you. I've owned Class A amps from 2 different manufacturers in the last 4 years, and haven't experienced that problem. As far as I know the Pass X-250 is not Class A anyway.
It sounds like you have a ground loop hum. You can read about Ground Loops on the Pass website. You can also go to the PS Audio wesite, look in the 'toolbox' and click on 'humbusters'. It's an interactive program that will walk you through issues like this. It will prompt you with questions and then take you to the next step in the troubleshooting process depending on your answer. It's simple and effective.

Good Luck,
John