Well, to demonstrate how strange causes of hum in one's amp transformers can be, here is the latest.

The high intensity lamp I have on my desk is the cause of my hum. I plugged it into a several different outlets on my several power strips. No change. I plugged it in by itself into a separate wall outlet (yeah, probably in the same circuit as the rest of the stuff) and no change.

Then remembered there is a circular switch on the lamp cord that provides a "high" "low" and "off" setting for the lamp. The "low" setting produces the hum. The "off" and "high" settings do NOT produce any hum. Voila!

Now all I have to do is get a lower wattage bulb so the high setting doesn't blind me.