mine use to hum badly until the nice people at aanvil but a new (bigger) tranny in it. in correspondance with bel canto it seems they replaced quite a few humming transformers under warrenty.
Its still a torroidal transformer - just bigger. The first one was 1300VA, the new one doesn't say, but apparently they are now using the same transformer in all of the evo's (at least thats what aanvil say).
I think the problem may be the use of cheap chinese transformers, but since the evo4 is only £3500 new, they have to cut corners
I was amused at the different approaches to amp support. So does it make a difference putting it on a fancy stand + audiophile granite + stillpoints or just hide it on its side next to the armchair so the misses doesn't notice it