When the music gets quiet, I could hear a hum/buzz. It's only noticeable when the house is quiet eg listening at night. The hum has some interesting characteristics:
(1) the hum can only be heard through the headphone.
(2) the hum can only be heard 5 seconds after the amp is switched on, and not initially (could be a safety feature of the amp to protect headphone I think)
(3) it gets louder as the volume is increased.
(4) the hum is there even if it's not connected to any RCA cable.
(5) the hum is noticeably reduced when rca cables are connected.
(6) the hum is present regardless of the wall outlet, or the use of a noise suppressing surge protector board.
(7) it's still there when hooked up to a portable CD player which uses a 2-prong power supply (whereas the amp uses a 3-prong).
What's your impression? Ground loop, mechanical, transformer or tubes? I'm not so sure it's ground loop as the hum gets louder with volume, whereas the typical ground loop hum remains constant (I think).
Any help is greatly appreciated! I have about a week to decide if I want to return the amp but I'd just like to make sure where the source of the hum is first.