what is the best Kt88 to get

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

I was tried deferent tube in my Prima Luna  HP.

Until I purchase KR-KT 88. I'll never look back!

@moose89 -

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

      No risk there.     Just be certain that the two new tubes are matched with one another, plugged into the same channel and properly biased.