A low hum is almost certainly going to be 60Hz (or 50Hz in some parts of the world) line noise.

As many have said, it’s often a ground loop - but since you have a single device plugging in to unpowered (presumably!) speakers, that’s not the problem. In your case, the stereo itself is producing the hum.

So if you’re in North America, there’s one thing to check before returning. I’m guessing that a cheap Amazon amp has either a wall-wart or two-prong plug. If possible, try rotating the plug 180 degrees and see if that helps.

However, there’s a good chance that the plug is polarized (i.e. one prong is larger than the other) in which case you won’t be able to flip it.