[macello-ita]

Hello, I have a Leben cs 300 amps for two years and has recently started to make a hum noise only on the right channel, the sound has a volume of about -65db, audible, especially with my high efficiency headphones, but the noise does not change whith the volume is always fixed, but is negligible when using the speakers, I can hear it even when I turned on the tape monitor, i have just try to change all the tube and try to connect the amp at the power conditioner (furman pl8) and change nothing, I also tried to shield the tube, and open the amp to see if there were any cables out of place or anything, but everything was in order, at least to know what the technician would check.
 
or What else can I try?
 
Sorry for the horrible English, but unfortunately it is not my native language. thanks