I just purchased a GDA-600. I know it's all relative, but it seems to have made quite a difference between my Apple TV and Adcom-565, 555II.
I do notice a very soft buzz/hum from the unit. Does anybody know if that is normal or is something wrong? The seller forgot to send me a power cord (which is on the way), so I just used an old cord from a Dell PC. Could that be part of the problem.