This looks like exactly the same problem that another Power Station owner and myself are experiencing and discussing here (Warning: long messy thread, where we eliminate causes for the hum and figure out things along the way):
I’ve now also found reports of the exact same problem on several guitar forums.
I think it’s pretty clear by now that this must be either a bad batch (not very likely as the affected units seem to have been purchased over a period of more than a year) or an inherent design flaw (possibly only with 230V units, as all reports I’ve seen about this hum seems to be coming from various European countries).
I think it’s time someone at Fryette take a much closer look at this, instead of trying to come up with alternative explanations for this hum. The hum is clearly originating from inside the Power Station, and will almost disappear if you remove the top lid. Then, if you move the top lid closer to the top of the unit, without actually touching it, the hum will gradually increase as you move the lid closer. And yes, the hum is present with no amp connected. The hum is 100% definitely originating from inside the Power Station itself, and almost certainly coming from the power transformer.
To me this strongly suggests a severe design flaw. The Power Station is utterly useless to me with this kind of noise level. Shipping the unit off for a repair (AGAIN…) obviously won’t fix anything if it’s a design flaw rather than a faulty unit. We need Fryette to step up and sort this out ASAP, as the situation isn’t really acceptable with this many reports of the exact same hum.
Personally I’ve already purchased a Tone King Iron Man II instead, as I quite frankly don’t see a solution to the problems with the Power Station anytime soon. I would still like to get it fixed of course, and I hope Fryette will come up with a solution so I can at least sell it with a good conscience. As it is, it’s really not worth anything to me.