I just got off the phone was Mesa. I called a number i recieved from them a week or so ago to get directly to a person. I left a message, and about an hour later I got a call back of him offering to answer all my questions. Mesa has stopped making 6550s, and Im in need of some. The man told me that I needed to find the most "rugged, durable and strong" tubes I could find, seeing how older 400s run off of 6 tubes and need stronger tubes. He had no suggestions. I mentioned JJ tubes, and he responded that Mesa once tried JJ tubes and did not like their quality or preformance and didnt sell them. All i hear about are people raving about JJ tubes, so I dont know if I should take that with a grain of salt or not. I dont really know how to look for a "rugged" tubeset, but maybe some of you guys here do. Thanks in advance.