Today is my 15th day with the A2 and I suppose I am keeping it. There is a slight hum, but it has subsided considerably and I would consider it acceptable. I can only hear it within about 5 inches of the top grate.
The amp sounds sweet with a solid and dynamic Class A sound. It reminds me of my Krell integrated. I think it is driving my HD650's well, to the point I feel I am limited by the phones and not the amp. I'm supposed to be getting some HE-500's and others to play with soon so I can see just what the A2 can do.
I only wish I had heard the A1 so I could compare it. I'm pretty curious about how the A2 sounds compared to A1, Lyr or Mjolnir.