I've had my Ampeg SVT-4 Pro head for a bit over six months. I purchased it new, and have been absolutely adoring it through my PR-410HLF cabinet. The only thing I've noticed that actually really bugs me is a hum through the speaker cabinet. I have checked every method I can think of to trace the hum, but nothing seems to make a difference. Here are some facts about it: 1) It is independent of master volume and gain. Even with one/both of these at 0, the hum is still there, and its volume does not change. I am not confusing this with standard amp hiss, which is almost nonexistent on this amplifier until the master volume is near full. This I am not concerned with, it's the hum that bothers. 2) It is independent of pedals/rack effects. This is sort of proven by number 1 too, but I had to check it anyway. 3) It is not because of the extra power outlet on the back. I have my Korg rack tuner plugged into that, and removing that plug did not make a difference whatsoever. 4) It does not depend on the electrical circuit. I've tried it on a variety of circuits and always the same result. The hum is the same volume and never goes away. It is not due to bad line power. So I'm left wondering why this hum continues. I cannot for the life of me determine where it comes from since it must be happening downstream of even the master volume control on the amplifier. Perhaps Jerrold Tiers could help here too. If anybody has any comments on this, or suggestions, please let me know.