How to pick which brand of 6550 tubes to purchase

Unless you don't play your amps very often, after close to 20 years of use on your old tubes, any of the better quality, compatible new production tubes out there will likely sound like a revelation compared to what you're hearing now (especially given around 40 hours or so of burn-in on the new set). I haven't tried them all, but the Svetlana 6550C's that Newbee mentions are quite smooth, extended, dimensional, clean'n'tight, and natural sounding, and have so far been perfectly durable for me. Nevertheless, I am also interested in trying either the Svet or JJ KT-88's at some point, which according to some might be a bit warmer, if not quite as tight and extended.

OK, now I've had one of my Svet 6550C's fail before its time was due. Luckily, I happened to walk back into the room and glance over toward the rig just as the tube began lighting up bright orange inside - I was cranking pretty loud at the time. I ran over and killed the volume and the amps. The music had actually continued to play without distortion right up until I doused it, but after allowing things to cool down and swapping out the light-show tube, the output fuse was blown when I tried to turn the amp back on. Fortunately, a new fuse took care of the situation - there were no blown resistors or anything. Why the tube went however, at only about 10 months old, I cannot say.