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.