Though Bob at Eurotubes may know his stuff too I am more inclined to follow Myles only because you can go get GT's nearly anywhere tubes are sold. Sure you can go get JJ's too at some shops but they are not always graded or screened. GT is just more convenient. Then again, I live in So Cal where I can get just about anything I want with a short drive. If you are relying upon ordering it is just a matter of taste. Then go with someone that has a rep: Myles, Doug, Bob, etc.
Greg, though others may test their tubes too GT has a good history with their screening process. I know that they can be bland if you don't buy into SAG tubes but then again you get what you pay for. That is a properly screened tube that has been rated. Using this rating you can tailor your amp. Like I said for me GT is convenient because of the numerous retailers here in So Cal. I would rather pay a few extra $ and have it in hand immediately even with a short drive than have to wait up to a week. Besides, GT offers just about any tube currently manyfactured screened and selected. Just my $.02