Switched left and right tubes, replaced rectifier with a totally different model, same result. When the WA6 is initially powered on, there's a loud hum/buzz on the left side, that drops off sharply after a few seconds, but when I turn off the amp, it's noticeable that there was still some hum going on, compared to the silence that I now hear. It doesn't increase with volume, and it can be masked with music. I'm using Grado RS-1i w/ the WA6 low impedance switch, so I'm still wondering what's causing this, and especially why just one side. I'm guessing the problem is within the amp, and so I should contact Woo Audio about this? Thanks in advance for the guidance. EDIT: Hm, flipping the impedance switch does nothing, either to the hum, or the volume. Is it possible that it's broken?