Just recently got a chance to compare the 3 pairs I have. These were in a mini-Major homebrew amp so it is an Ultralinear power section. 500vdc plates and biased at about 56mA cathode current per tube.
Gold Lion - The most balanced tone of the 3 with the sweetest top end. It had the least amount of low end but not by much, also the least amount of glassy/shimmery highs but again, the most balanced.
JJ - Big, round low end with a bit of extra sizzle on top. This is similar to my experience with JJ 6L6s relative to other 6L6s.
Penta KT88SC - Muscular bottom end and lots of top end detail but missing some of the important mids guitar players need. Might be good for bass or HiFi but based on this pair, not the best choice for a guitar amp.
Relative to the Gold Lions, the JJs sound a little bloated in the lows and sizzling in the top while the Penta sounded scooped (and not in a good way) and really glassy.