I just recently picked up a used Hartke 3500 head to use primarily for at home practice. I play active basses; Spector Euro 4 strings to be precise. I just recently noticed that, on occassion, when striking the E string really hard with a pick, there was a kinda farty distortion. It doesn't happen all the time, just when playing hard. So, I tried another cabinet....same thing. I changed the tube in the head....same thing. This only happens when using the tube preamp. The solid state preamp on the head is fine. Spector's Tonepump preamps are pretty "hot" and have lots of output. Could this have something to do with it? Is it simply that I'm over driving the tube and clipping because of the active bass output? Or is it a problem with the head? This doesn't happen with my main head....an SWR 750. Any help or advice would be appreciated. Thanks!!!