For Tube Folks...

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. Unless you are curve tracing, you will need to test under the conditions of your amp. Fixed bias amps usually allow for adjusting the balance of the bias current in the output transformer so perfect match is then not necessary. With cathode bias you will want to be closer. If a tube is going to fail from a defect, it will most likely fail in the first 100 hours. Also, in my experience it takes tubes 50-100 hours before they sound the best. For me it doesn't make sense trying to locate matched NOS output tubes. I would suggest finding current production output tubes that you like and stock up. And if the dealer burns them in first before testing I would consider that a good thing.

As for boxes, tubes from the 1950's & 60's the boxes used cardboard with a high acid residue that over time turns them a brownish color and makes the cardboard very brittle. Just opening the box to test can damage the packaging.  

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 ;-)