It helps if you use a (real) RFI meter to measure the EM field, too.Sorry. What I meant but did not express well is that natural magnetism can only be blocked by a Mu metal alloy that has been annelead after shaping. Electromagnetism can be blocked by any metal and, in all cases, the thickness of the plate/board does add to the damping effect on the magnetic field. So you could block electromagnetic interference with something as inexpensive as copper or even, honestly, a few layers of aluminum foil. You’re basically building a partial Faraday cage. Plus, the copper / foil costs a whole lot less than Mu metal all other things being equal. That’s all.
Only Mu metal needs to be annealed (heated then cooled without being shaped again) in this context because it has a dense molecular structure that gets “disturbed” by any and all metal work. No other metals that I know of need to be annealed after shaping to block electromagnetism.
It should / could, but you might need to use more than a few layers of it and possibly solder a ground wire to it and connect that to a “drain” ground post of some kind (or not). Please remember that aluminum as all metals conducts electricity. Plus, if the Mu metal is already cut to size with installation instructions and is not too terribly expensive, it might not be worth messing around with the aluminum foil because it’s not that great a shield compared to copper and (yikes!) silver and, yep, Mu metal.
It’s not unusual for some circuits to hum when there is no interconnect cable attached because the cable itself MAY be the only way that the amp is truly grounded, though, yep, there can be many other sources of hum.
I received my ISO Pucks this evening as well, and fortunately, they're going back because I found the issue with my WA33. It's not the transformers or a ground loop, or any of my tubes or upstream gear...it's the bottom cover of the amplifier unit (not the PSU unit) vibrating and causing my WA33 to audibly hum/buzz, traveling through my tubes into my dynamic drivers. I've never had an issue with driver noise using my planars. I applied even upward pressure along the bottom cover of the amplifier, and sure enough the amp is dead silent, and I only had a very faint noise through my dynamic drivers, which is normal for effiicient dynamic headphone drivers with the WA33.
