Get a digital V meter, something that reads down to 1V or less. Should be around $20.
Connect it to your speakers while you hear the humming. Measure the AC V.
Change speakers. Repeat.
I think you’ll find that the AC V is the same. If so, this means you are just hearing the
difference in speaker efficiency.
Here’s a good meter with a 400mV range that would be perfect.http://amzn.to/2bISae5
By the way, this works because almost all meters are sensitive to 120Hz or lower, which is usually where power supply and ground loop hum lives.