Issue #19 of the Vacuum Tube Valley Journal has a review of 6550/KT88/KT90 tubes.
http://www.vacuumtube.com/
They ranked the sound from each variant in an Air Tight Amp. Unfortunately, I can't find my copy to give you all the results, but this was a very informative article. There were a couple of current production models that rated very well. One of these is the JJ KT88 for about $50 each.
I am still using NOS GE 6550A's so I can't give you comparison form my own experience, but I will try the JJ's when the time comes to retube.
Try to get a copy of the Journal...or maybe someone remembers the complete results and can relay them here.
http://www.vacuumtube.com/
They ranked the sound from each variant in an Air Tight Amp. Unfortunately, I can't find my copy to give you all the results, but this was a very informative article. There were a couple of current production models that rated very well. One of these is the JJ KT88 for about $50 each.
I am still using NOS GE 6550A's so I can't give you comparison form my own experience, but I will try the JJ's when the time comes to retube.
Try to get a copy of the Journal...or maybe someone remembers the complete results and can relay them here.