You have probably already check these, I find using speaker cable where a line cable is suppose to be used is a common reason for buzz. Next try to isolate where the buzz is coming from. Unplug everything from your power switch and just have the main power amp on and the speaker cable that goes to the speaker connected, everything else unconnected, switch on the power amp. If there is buzz with only your power amp you know that either it is the power, power amp or speaker cable. Most likely it will not start buzzing at this point. Shut of the power amp and plug one thing in at a time, turning the power amp on after every new piece of gear is cabled and powered up. You should be able to isolate the issue, sometimes it is simple like the wrong cable used, others it is power conditioning such as a dirty ground, sometimes it is low power conditions or high power conditions... Also check if isolating ground of all your gear, I.E. hum frees on your rack gear, so they aren't sharing case ground.

If none of that works, you may want to teach it the words.