I just bought a Thunderfunk TFB420 from a very reputable dealer who sells used equipment. I have not had a chance to talk to them about this since I just got my amp today (the first really excellent service I have ever had from Fed Ex Home Delivery!) and they will not be open again until Monday. This is a used amp. It looks to be in really great condition. But dang! This sucker hums like crazy! Not like it's in the preamp or amp. There is an immediate hum on power up then you hear the speakers come in with the same hum at a lesser volume. If you turn the amp up the hum does not get louder. It definitely sounds like a component isssue. The amp sounds great by the way. One of the best plug-and-play tones I have ever heard. My only concern with the amp is this hum. I have heard contactors hum like this at work but I am pretty sure there are no contactors on the amp...but I could be wrong I guess. Maybe a relay anyway. OR a transformer. I A/B'd it against my GK700RBII and my Rack (RBI + PLX2402) and it is significantly louder than either of them. Anyone else run into this kind of problem? Is it common? Is it O.K.? Any help would be appreciated. Thanks! Joe.