Check all your grounding and across all your solder joints with a meter. Loosen and re-tightnen all your jacks and pots to the chassis. Remove the mods. Go through the whole circuit, take you all of 10 minutes. Try another guitar cable and plug it into a different circuit in your house. My Vibro champ is dead quiet with nothing plugged in and the volume on 10. I worked on a champ with similar symptoms. Turned out the ground wire on the newly installed 3 prong cord was hanging by 1 or 2 strands on a grainy blob of a cold solder joint.
Sometimes it becomes a process of eliminating things one at a time.