Ducati - Talk to the guys at Doug's Tubes. I went from stock Sovtek 6550s to JJ/Tesla KT88s and more recently to Shuguang solid plate KT88s. Both mftrs. KT88s were better sonically than the Sovteks in several ways. Howevere, the Sovteks were ~5 yrs old or so and I'm guessing not even in the league of your Phillips 6550 to start. I really, really liked the JJ/Teslas but they were not too reliable. Compared to the JJ/Teslas, the Shuguangs seem more detailed, with more hi end extension and air. Downside is a bit of harshness on some CDs(but not vinyl) when listening at louder volumes. Good luck with your decision.
