I'd been using Phillips 6550's in my Cary SLM-100's for a few years and thought they were getting a little weak. The prices of NOS anything(let alone matched quads/octets of anything NOS) nearly left me apoplectic! I tried an octet of EH KT88's(cryo'd), and couldn't get them out of there fast enough(they sold pretty quick on eBay, thankfully). Then I contacted Kevin at Upscale Audio, and he recommended the latest Winged "C" 6550's from the ST Petersburg plant. I was very pleasantly surprised that a newly manufactured tube could perform as well(transparent, dynamic, liquid, extended) as the NOS tubes I'm accustomed to. If you're not familiar with his testing/matching/burn-in procedures, check them out here: (http://www.upscaleaudio.com/product.asp?itemid=16&catid=85) I have no connection to his company, outside of being a very happy customer.
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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.
I went from Sovteks KT-88, came with my recently purchased Quicksilver V-4, to Genalex Gold Lion. This switch had the same sonic effects as when I replaced the EH 6922 with Amperex 6922 U.S. PQ in my CDP, from thin and airy to full, detailed and dynamic!! I bought matched Quads from the TubeDepot they were great to deal with and fast (no affiliation) so far no problems!!
Genalex Gold Lions are used in my MC275s run in mono. Data sheet says two valves in class AB1 gives a continuous output of up to 100W.
I've used the following to retubed my MC275s and C500T:
[url]http://www.tubeworld.com[url/] is another great source.