Tubes are supposed to last for YEARS.
Are you in a hurry? I suggest you buy a full set of cheap used tubes, so, for the rest of your life, you don’t live without music, while searching/deciding on the next set you will use. Time is a luxury, time for research, find most comfortable source, wait for a great price .....
2 matched pairs?
from your stock? or buy from a source with matched pairs ready to ship?
If they have matched pairs, they have a tester. Perhaps, for a small premium, they will test and find two pairs for you, if you can wait.
Ignoring all the other electrical measurements, for me, it comes down to left/right balance.
I just lost left channel, was a blown 6sl7. Happily, when I searched my boxes of randomly acquired tubes, I had two GE 6sl7’s, used, in my collection. I replaced the blown one, oh happy day, and, l/r balance was terrific. Time to search ....
GE 6sl7 is short, so, I decided, while waiting for the two matched pairs I ordered, to put the other short GE in right channel. Nooo, right channel volume way down. Original tall one back in, l/r equal. No way a matched pair, but ..... especially temporarily.
btw, I had 4 look alikes, 2 6sl7 and 2 6sn7. preamp tubes before KT88’s. I mixed them up inadvertently not long ago, left channel was weak, yanked for testing, realized they were different. They look identical. I purposely ordered chrome top 6sl7’s so I will never mix them up with the 6sn7’s.
I have my own tube testers. The little cheapo one always gives the same relative strength answers as the big one.
not to buy this one, but this is the modelhttps://www.ebay.com/i/323712928966?chn=ps&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&itemid=323712928966&targetid=858380238160&device=c&mktype=pla&googleloc=9003512&poi=&campaignid=6470719340&mkgroupid=77931583832&rlsatarget=pla-858380238160&abcId=1140476&merchantid=115026223&gclid=CjwKCAiA98TxBRBtEiwAVRLquwrExfEjEkByy2tj4bjcKQGCxdmf1PYLG13wl2I98eSMoSUf7BXZoRoCMOsQAvD_BwE
I love this little thing.