I really liked them. I was heartbroken to find them not being produced when it came time to replace them. I had tried JJE34Ls (had pin problems), 6CA7EHs (Paul Grzybek's specified original tubes - they were "okay", imo), JJ/Tesla KT-77's (garbage right out of the box), and finally found Tung-Sol 6550's (current production) which I enjoy immensely. So much so that I'm on my second quad from Upscale.Those SED 6550s were great tubes! I can remember replacing many Chinese KT88s and 6550s in the 90s with the rock solid SED.