what is the best Kt88 to get

For current production, the KR KT88s are incredible.  They have a higher plate dissipation than a typical KT88 and can be driven harder without compromising tube life.  Wonderful detail and great frequency extension.  I have them in a pair of Art Audio Opus 4s.  
I have no POV on NOS tubes.  

               The KRs are certainly pretty!

https://verdantaudio.com/products/kr-audio-kt88-each

does the KT88 effect the sound as much as the 6sn7's do?

can’t give a useful answer without knowing what the amp is

I have had good luck with Gold Lion.  I can't even tell you how old the ones I'm using are....they are a back up quad and they still match very closely.   I have had other type GL like 66 and 77 and they didn't last nearly as long 

I just received a Cary SLI-80 amp. It has (2) Genalex Gold Lion tubes with a mixed quad. They have “brown ringlet”circles on them? Why?

can I risk damaging my amp by purchasing (2) more to make (4) KT88 GL quad?

I don’t have a tube tester. Does it matter?

I have a Cary Bias meter. Thanks