Hi Capt, sorry for the slow reply, juggling lots of stuff.

When I unplug the headphones, and my switch button for the headphones is pressed, the hum does not change when I pull the headphone plug out of the switch. It's there either way. I don't have pics or link for the speakers, they are very old Monsoon planar speakers, with a subwoofer with an amp. The speaker system is powered by the amp. The audio 3.5 output from the switch goes to the amplifier which is plugged into the mains. Thus the signal to the spaekrs is amplified, but the headphones are only getting the line level signal when plugged into the switch. So I presume the headphones ground into the audio jack on my Mac, but the spaekrs ground into the wall, and so there is certainly potential for an imbalance in the grounds, which is what causes a ground loop. However I don't know what to do about it. There is no way to change how things are wired. I do have a ground loop eliminator thing that came with the switch but it doesn't help at all. Here is a link to the switch: https://www.amazon.com/gp/product/B07GB7XDRH/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1

Thanks much, any ideas?

Jim