Beautiful amplifier. Have you contacted the manufacturer? I have a large tube power supply on my tube phono pre and I can only get the pair hum-free by distance and orientation. You could try a grounded (mu?) metal plate. Have you ruled out overly sensitive tubes?
Proximity Hum And How To Eliminate It
I own a phono stage with separate power supply, whenever I put the power supply near the phono stage unit I get 60hz hum caused by the proximity of the 2 units being close to each other.
Previously I could avoid this because I was able to locate them far enough apart. Because of circumstances I now have to place the units close to each other.
Does anyone know how I can eliminate this proximity hum, either by shielding them from each other with something or????