Hi. ^+1 the devil is usually in the details . You can obviously limit the sources a bit by testing the amp/speaker without the cord plugged in. The treble or high frequency can't control hum at all, are You sure it's not hiss you're describing? Usually; hum=low
feq (mains etc.), hiss=high freq (RFI etc.) All the amps hum and hiss to an extent, it's not unusual. ...