A note on this: The idea is not to shield the PT as that would require an entire steel case around it. The idea is to use the steel to shield the parts that are being affected by the PT's magnetic field. In your case, I would start with the capacitors you mentioned as contributing to the hum. I just posted about this on another thread here recently. In that case, the DUT was a commercially made "Pultec inspired" Eq, a good one. I will mention again that you need to be extremely careful moving a piece of conductive metal around inside a chassis where High Voltage is present (Note: the steel does NOT need to be connected to ground, however...the shielding comes from absorbing/disrupting the magnetic field via the ferrous properties of the steel, not draining it off to 0v). In cases where there is not a lot of room inside (although you have plenty) and also to try and narrow down what's being affected, I have sometimes just used a steel tool that's ready-to-hand, like a large pliers or an adjustable wrench to hold near various parts and see/hear if the noise comes down. Once you identify the area, you can come up with a way to do a permanent shield, like mounting an angle bracket to the floor of the device.
One more thing: Have you tested this with the steel lid screwed firmly in place, or have you been waiting to fix the issue before you close it up? Hint...the lid may fix the problem...
