You'll have to do some testing to see where it's coming from, but if you remove the junction box and the hum goes away, then doing some modifications on the junction box should fix it. Replace at least one pair of jacks with plastic bodied jacks so they don't connect to the chassis and short to the other jacks. Neutrik and Cliff make good ones.
Or, just add an isolation transformer as mentioned. If you're in North America, the Hum-X is a safe way to lift the power ground on one amp and has less effect on tone than an audio isolation transformer would. (They only make a 120V version as far as I know)