I am answering here in the hope of helping others and hopefully influence Presonus future speaker design.
The Hum in Presonus Speakers is due to a noisy power transformer.
I have had this experience on a new Eris 4.5
pair bought in 2020 which I am unable to return for refund or exchange to the UK supplier or Presonus Europe.
They are good monitor speakers that need a quieter power transformer to be sourced to replace the current model.
Disappointed in Presonus after using their Software for the last 5 years.
A studio monitor speaker should not hum at zero volume with no inputs and this is a poor design that can be rectified. I would be happy to pay to replace the transformer if an improved suitable version was identified.