For Tube Folks...

I still own a quad of matched GE 6550’s. I have them because they are a superb sounding tube and quite robust. I removed them from a Melos tube amp that I bought new and that used these as the power tubes. Replaced them after just a few hours use with a quad of KT88’s, which gave me a little more power.

If the GE 6550’s that you bought work well in your system, then you have some of the best power tubes out there. I went through a number of tube amps in the interim years, some with Winged C 6550’s, still have the GE’s to this day..if that tells you something.

If you want really well matched output tubes you need to burn them in for awhile and retest. They will drift around a bit when they are new.

Referring to what @john_tracy typed above, the second time I retubed my V12 I went with a couple of sextets of EL34s that Jim McShane sold me.  It took a while to get them, and every couple of days he would send me an email telling me how the matching was going.   I think that was sometime in '20.  I set the bias and for the longest time it stayed where I set it on both sides.  

On the other hand, the first time I retubed that amp in 2018, I bought two sex tets from Viva and they arrived quite quickly.  So I don't know if they keep a lot of tubes properly burned in and matched, or if they don't put the time into matching them that Jim did.  I do remember the biases doing some goofy things with that batch of tubes.  

@runwell Keep the new tube manufacturers in business because at some point in time, NOS will no longer exist. Some well reviewed newer tube brands out there; Grant Fidelity Special Edition, Linlai Elite, Apos Ray Reserve.

@daveyf

Thanks for your encouragement regarding the quality of the 6550 power tube. I appreciate hearing of your first-hand experiences.

@goofyfoot

Thanks for the names of the 'newer tube brands'. I have not heard of them.. I hope they prosper... but not too much ;-)

Actually test data is very important but doesnt tell you if the tubes are lightly used or not. It is not unusual for there to be up to a 15 to 20% difference in the test readings on NOS tubes of the same type and manufacturer. So you can actually have lightly used tubes which can test higher than true NOS. 

As Ralph points out, the tube pins are probably the best indicator of use. However, if the tubes were tested then there can be vertical marks on the pins. Heat marks/discoloration can also be an indication of use. Problem is that the process of manufacture wasnt exactly as we expect today and variations on things like getter flash, clearness of glass, etc are not uncommon.