So how would you compare the EH KT88 to the TS KT120?
Have you read any reviews on the GL KT88? There is a ton of information out there. Check out Upscale Audio as Kevin's site has quite a few reviews on the tubes. I have never used Gold Lions but from what I have read they are very good and probably more refined than EH.
I have the TS KT120 and find the upper mids are a little harsh or energetic so I rolled in some Mullard's to tame it down. So yes you can change the stock 6SN7's to tailor the sound to your liking. I haven't bought any 6SN7 for a couple years but the CBS Hytron and NOS RCA grey glass were very good (The earlier the better). Personally I would try this route 1st before changing the output tubes unless you aren't satisfied with the overall "tone" of your system then I would change the output tubes.