Does the JSX have a ground switch on the back? See if it does anything for you.
Also, are your outlets all properly grounded, 3-prong?
It may be a tube issue also, none of them are visibly redplating or anything, are they? They may be too far out of spec with each other.
Though, the buzz sounds more like a switchmode power supply to me than it sounds like normal amp hum.
You aren't resting your computer or hard drive on the amp or speaker cab are you? Or anywhere really close to any of the signal to and from the amp?