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
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