I have owned a '78 model master volume Marshall loaded with 6550's that I thought sounded thin, trebly and buzzy-fuzzy in the distortion department. I owned a '72 JMP super lead that sounded terrific, I should have kept it and had it converted over to EL34's. I also had an Eighties model JCM 800 master volume loaded with EL34's that I thought sounded great for hard rock and heavy metal. The best sounding one I ever owned though was a 1973 model 50 watt combo 2x12 model that I took out of the combo cabinet and into a seperate head cabinet. It was a non-master volume model loaded with EL34's and would start to clip with the volume turned to 3. Sounded just like later Zeppelin or early AC/DC.