Hello all, I having an issue with one of my DSL40C amps. I have a tech check for circuit board issues and there doesn't seem to be any, at least the best he can tell. I have been able to duplicate my issue twice and still the tech can't figure out the issue. So I purchased the amp used, when testing it I found a microphonic power tube. After getting the amp home, I purchased a new set of preamp and power tubes. after installing the tubes and setting the bias. The amp sounded very musical, harmonic and very playable. after about 3 to 5 hours of use, not all at the same time, I noticed as if someone flipped a suck-it switch and without any pops or noises the amp went to sounding extremely harsh and gritty. I checked all connections and pulled the amp apart to check the bias. The bias on one tube almost doubled in reading. I re-biased the amp but it still sounded awful. I lived with it for a few weeks and thought maybe I had gotten a bad tube, so I installed another brand new set of EL34's, again the amp sounded great, but then again after around 3 to 5 hours of use it went back to sounding like crap. I'm about to pull it apart again to check the bias. I think I have an idea of what the problem maybe, but I wanted your input. Thank You!