Sounds like a ground loop. Is your source balanced? If not using unbalanced to balanced Adapters just reduces the gain on the speaker (thus lower noise) but you still have a ground loop.
Thats more correct true, @antcollinet described it pretty good in another Post too
What I wanted to say is that on a higher ohm speaker you need a bigger voltage supply to gain more max power, while on a lower ohm speaker you are most likely current limited first and wont gain much from going...
higher Ohm need higher voltage because theres more resistance.
you can see that in amirs review on page1 too.
most amps struggle with lower Ohm because you need to push more current to achieve the same power which means more heat