I have a pair of the Eris E7 XT monitors and I can confirm that they have a very annoying power-mains related low frequency "hum".
I have digged into this in order to find the root cause and see what can be done in order to get them to work without any kind of low frequency hum.
I found that the reason for the LF-humming is due to the mains transformer which power the internal amplifiers. The transformer is of the "conventional" E-type (not a toroid transformer) and therefore emits/leaks a significant level of magnetic field outside the transformer. The low-level signal conditioning electronics, which are implemented on a PCB of its own (carrying the input connectors, potentiomters etc), picks up some of this magnetic field and translates it into an electrical signal, the HUM, which is in sync with the power mains frequency. 50/100Hz in my case. This signal is then fed into the internal amplifiers (one for the woofer and one for the tweeter).
Unfortunately, there is no easy way to get rid of this magnetic field nor to shield it off, as the design/implementation is what it is...
On one of my speakers, so far, I have split the complete internal amplifier module into two separate parts. One containting the power transformer with the mains connector, voltage selector and mains switch and the the other part is the 2 internal power amps incl. the input signal board, rectifiers, voltage regulators and heat sink. The transformer part can now be put in a greater distance from the internal electronics (in a separate box). I now have to use an electrical 5 core cable to connect the two separated modules, carrying two "low-voltage" AC-voltages and a common AC ground reference. all taken from the secondary (safe) side of the transformer.
I can confirm that the result of this is a complete noise free monitor without any hum, working as expected.
I belive that this mains hum problem, must either be a design glitch by the Presonus design team or a painful mistake by the manufacturing team, selecting a low quality transformer, maybe with a poor iron core or a bad coil winding principle, producing a too high magnetic leakage.
So, it would be nice if someone at the Presonus team gave their view on this matter.