I did a search and got many results but no real answer. Wondering if something new has come up. I just bought a brand new SVT 4 Pro. It's a great amp. I'm into building DI boxes and I'd like to think I know a bit about ground loops and all that. I'm getting good results with a Counrtyman and a few other DIs out of the 1/4 inch line out or the effects send (that's a little hotter) but the xlr still buzzs. My first thought was to build a lifted xlr cable and not connect the shield. That didn't change anything. I lifted it at the wall too. No change. From what I've gathered, there is no ground lift switch because it is already lifted. According to my multimeter, Pin 1 does not touch the chasis. There's buzz out of the xlr out when the bass is unpluged and the mute switch is muted. Kind of a pulsated type buzz. Not really a hum. I'm running the xlr out into a simple Behringer mixer and some M-Audio powered reference monitors. It's my "real life job testing setup station" Everything else I own is quiet (or was made quiet) through this simple set up. When I use the pad knob on the front panel, it gets worse. The hum ratio sounds the quietest when the pad is at full volume. It's not the bass (I have several, passive Fenders) and the cables are all solid. It's actually a little better using the "post" button which I find somewhat unusual. The 1/4 inch out right next to xlr out sounds fine. The noise ratio/level there is what it should be. It must be the actual transformer or a loop created from the pad knob on the front? I feel like I've got a good handle on these problems and I've run out of options. Just a bad design? I've noticed others having the same problem. Was a solution ever discovered? I'm %100 convinced it's a design flaw or a common, internal problem. I guess the solution is to use a DI box but I don't really see that as the real way to fix what should be an air tight product.