Back to the starting point.

I replaced the two 9800uF electrolytics in 2 of the amplifiers. I moved the signal cables away from the A.C. cables inside the amplifier. I mounted the amplifiers in a metal relay rack and plugged them in without a ground cheater. A.C. and grounds are connected. I patched cables from my unbalanced surround sound decoder, which has common grounds, to the unbalanced input of the 2amplifiers. Hum was still really, really bad. It was too bad to listen to anything. I purchased the EBTech 8 channel Hum Eliminator plugged it between my surround sound decoder and amplifiers and the hum went away. I now have a very clean sounding left, center, right, sub-woofer system. When I get a chance I will change the electrolytics in another amplifier and mount it in the relay rack for my left and right rear Altecs.

I read a 12 page article about ground loops. Near the end of the article the author said "May the force be with you". Ha! Ha!

I have spent hours, days, weeks and now months working on this problem.

My three front channels - Altec 817 cabinets, 515E woofers, 805 horns, and 288K drivers. My left and right rear - two 820A cabinets, 515E woofers 511E horns and 288K drivers.
My four sub-woofers 8182 cabinets with Altec 18" speakers with new accordion cones.

It was worth repairing the amplifiers and getting the hum out of the system.