I've been a very happy AU-717 owner for the past 13 years (thank you, AK forums, for that excellent recommendation). I've recently started to detect an audible hum through my speakers whenever the amp is powered on, even with the volume pot all the way down. Changing sources doesn't change anything, nor does toggling the integrated/separate switch on the back of the amp. Any thoughts on what might be causing this? I had the amp worked on right after I bought it (by AK'er merrylander), but it's been so long that I don't recall exactly what he did. I just know that it has sounded really nice to my ears for all these years (and continues to... except for this darned low-level hum). If this is something that calls for review by an expert, can anyone recommend a technician in the DC metro region? Thanks for your help!