Well it's no longer an issue, but I wonder if anyone can explain why this happened. I built a JTM45 clone with a Metro kit, and I asked this question on the Metro board, but no one was able to answer it. I had excessive hum. Way excessive. What I finally traced it to was that the chassis was not connecting to the shielding in the bottom of the head cabinet. But here's the weird thing: It sounded fine with the chassis laying in the head cabinet, but when I bolted it down, the hum would get really loud as soon the as the bolts started to push the chassis down onto the bottom of the head cab. My solution was to put some additional pieces of shielding in the cab that lay on the cabinet shielding, and intersect and run perpendicular to the ends of the chassis. The ends of these shielding pieces touch (are actually under) the ends of the chassis when it's bolted down. But why would it not hum before it was bolted down?