I did some additional troubleshooting to try to isolate the cause. The hum sounds like a 60hz hum (maybe 120 hz).
Here is what I tried and the results (just listening).
1) Rechecked the amp with nothing plugged in to get a frame of reference. Maxed controls (tone and vols) in several settings. The more they were turned up the bigger the white noise (different that the hum I want to minimize).
2) Plugged in only the cable (no instrument). Got the hum... it of course increased as vol was increased.
3) Plugged in the strat (not playing it). Got more hum. Same results... hum vol increased as the amp vol increased. The hum was there but changed pitch slightly when I switched between neck, middle, and bridge pups. When I switch to the Neck/Middle pup position, the hum reduced quite a bit. I am assuming this is because the bridge/neck pups are wound in in opposite direction of the middle pup to reduce hum.
I am beginning to believe the source is the guitar and cable picking up some sort of emf or other signal.
Am I wasting time on this issue? I have read other posts that are leading me to conclude that it is a common problem related to environmental interference, cables and single coil pups.
Can anyone provide some insight or opinion?
How have you dealt with this issue?