Amp Trauma Pt. 4: return of the hum

I had to replace a mute switch in my pre-amp which had gone bad. I replaced it with an identical switch, hooked the unit back up in the system, and I am suddenly back to square one. The hum is absolutely mind numbing. I fail to see how a new switch could create this problem. The switch operates a relay, which in turn mutes the circuit. I double checked all the solder points, made sure I had at least one good tight wrap around all the blades on the back of the switch. Two of the blades are steel which you really can't solder, but the OEM was the same, and it worked. I am not sure how the switch operates relative to the circuit, but it appears to do it through the ground. This is most puzzling. Any thoughts are welcomed!

looking back over your previous dozen posts or so, seems like each time some change you have made has made a difference, either for the good or the bad, and almost all of them are totally unrelated, so it's likely that changing the switch isn't actually what has now caused you to have hum again. You mentioned some bad interconnects before, or it could be that ground on the input jack again, or it could be a dozen other things. The mute switch in the preamp is simple connecting the output jacks of the preamp to ground, which is like having a shorted input on the amplifier, and it that eliminates the hum, the problem is not your amplifier.
Is your input source, your preamp, and your amp all grounded to the same AC outlet?