Seems like any equipment in a metal case is protected from rfi

from Wikipedia:

Mu-metal objects require heat treatment after they are in final form—annealing in a magnetic field in hydrogen atmosphere, which increases the magnetic permeability about 40 times.[4] The annealing alters the material's crystal structure, aligning the grains and removing some impurities, especially carbon, which obstruct the free motion of the magnetic domain boundaries. Bending or mechanical shock after annealing may disrupt the material's grain alignment, leading to a drop in the permeability of the affected areas, which can be restored by repeating the hydrogen annealing step.

Pop quiz - What's the difference between heat tempering and cold tempering (Cryo)? Free sheet of Morphic Message Labels to the winner.

Geoffkait:

My answer would be: The temperature at which it is done. Please send asap.

That's very good. The Morphic Message Labels are in the mail.

RFI/EMI is a tricky beast to handle.

It is nearly impossible to make an enclosure 'tight'. As a former designer of military electronics that were required to survive EMP, it's quite a challenge even at much smaller audio scales. 

Thing about EMI/RFI susceptibility re: audio is all electronics - tube or SS - can convert very high frequency noise right down into the audio band via diode demodulation. And the higher the frequency, (like the microwave from cell phone systems) the harder it is to deal with.

Top issue for EMI entry is cabling to/fro the chassis - these are the real problem children. RFI walks along the cable skin and enters easily to your gear's nice metal box via audio connectors never designed for susceptibility control. Once inside, the RFI radiates all over using internal wiring. Even shielded cables, - although better than open wire - are nearly transparent to RFI at very high frequencies.

Re-engineering existing equipment is virtually impossible as to get real, testable results.