I figured out something that might help some of you!
When I first got them, I powered them on, and they made this whining high pitch sound for a few seconds. Then, that sound disappeared leaving a soft hiss that is normal for powered speakers. That hiss is not dependent on gain and that is the amplifier. The same behavior happened once I connected a source to the Aux using the 1/8 cable. Ok, great, totally normal!
I left the 1/8 cable in the aux with my source connected and then connected the 1/4 balanced cable in the back. Once I set up the new source, when I powered it on, there was a noticeable buzz, with a tiny bit of distortion. Once I removed the cables in the back, or the aux cable, the buzz vanished!! So essentially, it can only have one source at a time (it also says that in the manual but its not very clear, as it sounds as if you can only have one source in the back and that the aux is not part of that limitation).
Today, I powered it on with only the 1/4 cable input in the back connected and had the same buzz so I thought they were broken.... but I realized my iphone was connected to it via bluetooth. When I unpaired it, the buzz vanished!
In short, its probably by design to save Presonus money. Although you are not supposed to have more than one source as stated in the manual, it can create noise when the amplifier is receiving more than one input, wether it is via cable or via bluetooth. Make sure you only have one source connected and see if you are getting that buzzing sound.
Hope this helps!