Before you do anything else, take the amps off of the stands and see if the buzzing stops.
If it does, buying power cords, speaker cables and/or interconnects might not help is the buzz is a vibrational issue.
My isolation transformer buzzed louder the harder it worked, but it buzzed more when I sat it on a maple block of wood. when I placed brass points under the transformer as it sat on the wood, the extra buzzing went away.
Throwing money at cables is like taking anti-depressents for depression. They may or may not work, because there's an actual reason that a person is depressed, and it's not being addressed. If your amps are buzzing on the stands, there's a reason for it that should be addressed.
See what the difference is on and off of the stand before you try shots in the dark with things that may be unrelated.
Also, are the screws or whatever in the rack as tight as they should be? If you like the sound better with the amps on the rack, have you tried placing a weight on the top of the amps to add some mas and dampen the vibration?