I'm a lot closer to the source of the problem now. I was able to get rid of the buzz completely. Here's what further investigation revealed... It gets much weirder!
I think it's some kind of ground loop effect. It only happens a bunch of conditions are met. Sorry, it gets a bit complicated, but here goes...
Condition #1:
Stereo is plugged into a certain outlet. If I move it to a far away outlet, buzz disappears.
Condition #2:
I have a laptop, which I connect via DisplayPort to a monitor. Monitor takes digital audio signal from DP and converts to analog audio for output on a 3.5mm jack. When I connect this analog audio to the AUX input on the stereo (+Condition#1 is met) then I get buzz. Even if I connect the laptop headphone audio to the AUX input there is buzz, but only while the external monitor is connected. Once you take the monitor out of the equation, laptop headphone audio is fine. For any other isolated audio device (ie smartphone), the audio is fine.
Since this is a cheap chinese power supply, it may have some poor grounding design. In the stereo, it's a cheap 1-sided phenolic PCB, but the traces did look like they used star grounding. I might try probing around with a grounded lead to see if that makes any difference.