I've noticed a pattern. Whenever I enjoy the sound with GE tubes, I like the sound better when I use a different brand of the same tube type. I've heard this with the 6sn7, 6sl7, 6gbg, and now 6k6gt. I've switched over to Tungsol 6k6gt tubes and everything sounds better with them. Better bass, bigger soundstage, smoother, "sweeter" etc.
Like a lot of other tube types, the innards of the tube changed over the years and there was a lot of rebranding. I've got some Sylvania 6k6gt that look a lot like their later 6v6gt:
There are a bunch of different brands that look the same, I assume they are just rebrands. For the Tungsol I opted for the old school Tungsol look. I've never seen a tube that looks like this that isn't actually made by Tungsol:
Since the price is about the same I can't really recommend the GE tubes. I'll be trying some older Sylvanias and some RCA in the coming weeks. I also got some Mullard and Cossor 807 in as well and I'm looking forward to hearing them soon too.