I've noticed since beginning to try and resolve this "buzzing" is that if you touch the case of the Audya the tone of the buzzing in the speaker changes... that doesn't seem right.
It sounds to me like you may have a serious AC wiring fault (NO EARTH) at your home/venue. If you get a slight buzzing sensation through your fingertips as you brush them gently over the metal exterior of the Audya (but it disapears if you disconnect the AC plug) then your AC wiring is faulty (no earth) and you should power everything off, disconnect all of the plugs from the AC wall sockets and get it fixed.
It may just be a faulty extension cable. Try plugging both Audya and amp directly into THE SAME wall outlet or better still buy a wall outlet tester (its a little AC plug with 3 LED's on the rear and costs just a few pounds). But please be carefull if your wiring is faulty it is dangerous.