I thought EMI is what causes 60Hz hum? Anyway, none of this explains why the 3 guitars only hum with the HD500. Only difference is the external PSU. You might get the same hum if you bring those guitars close to the guitar amp PSU. I once discovered a laptop PSU in proximity to a large TV created hum through the TV speakers. Moving the PSU around reduced the hum to some extent. You sure it's not a bad guitar cable, or faulty guitar jacks?