Noise Through Speakers even when Helix Floor Unit is Off

Recently bought a Helix Floor unit, and I've been having a problem which I cannot find examples of so far. I would be very grateful if anyone could help me understand the issue, or whether I have a defective unit.

When I plug in the Helix into PreSonus Eris speakers through either the 1/4 inch outputs or the XLR outputs, and have the Helix plugged in to mains power, I get a lot of electrical noise through the speakers. I do not have to have the Helix unit switched on for this to happen. If I take the power lead out of the back, the noise stops. I have the speakers and the Helix unit plugged into the same power extension lead.

I have plugged my Scarlett 2i2 audio interface into the same speakers to test, with the same 1/4 inch cables and get almost no noise, whether the Scarlett is on or off (powered via USB).

I do not get noise through the headphone output.

So, I think I have eliminated the speakers or the cables, I wonder if you have any thoughts?