how brief was the outage?
When the power returned, was the amp on?
Have you tried switching the amp off? Not by turning it off with a front button, that's the stand-by. There is a switch in the back of the amp. First turn the amp into stand-by with a front panel button, then flip that magnetic breaker switch on the back of the amp down. By doing that you will let the amplifier power supply capacitors completely discharge. I would also take the power cord out of the wall after the magnetic breaker is turned down. It will take hours to completely discharge. I'd leave it like that overnight or for about 8 hours. You'll see the blue LED stand-by light on the front meter still will be lit even after you flipped the back switch off. That's normal because the caps are still charged and the LED will go out in a while.
Then with your CD player off, flip the magnetic switch on the back of the amp up and let it charge for few hours(2-3 hours should do) and don't play music. After about 2-3 hours turn the amp on and play some tunes. Then power down and see if the problem went away. If not, I'd recommend calling Pass customer service.
Also, I don't think you should hear any hum from the amp anyway. I don't get it with my X250.5 and your X350.5 should not do that either. Talk to them about the hum. It may be your cable TV near by your system and you may need a ground loop isolator. But mention the hum noise to Pass tech as well....see what they say. Please keep us updated as I am very curious about the outcome. thanks.......