what happened to Golden Tube?

Pretty sure they were gone by the late 90s or early 2000's.

Are you referring to Golden Tube Audio?

I have a Golden Tube SE40. It has three 6L6's per channel. PSE (Parallel Single End) for about 20 watts. Which makes it quite powerful for a single-end tube amp!

Yes, I think the SE-40 was pretty much the anchor of their line-up.  Just feeling nostalgic and wondering what happened. At their price point they were pretty highly thought of.

I have owned GTA amps and preamps.  They got a bad rap because they used some low quality parts and they exited the scene rather quickly in the early 1990s.  But having said that, their designs were pretty good and the units sound pretty good.  I have rebuilt (or had rebuilt) every GTA piece I ever owned and for a modest price the mods made them sound great.

The SE-40 is a very decent tube amp, 40W at 8Ω and reportedly 80W at 4Ω (I don't find that credible).  5881 power tubes are spec'd but you can use 6L6GCs if biased properly.  If you find one of these make sure you get the pigtail biasing cable or you will need to make one (easy).  These make wonderful monoblocks if you are willing to do a little work.  There was/is a shop (Sonic Craft) that specialized in SE-40 upgrades but honestly any good shop can do this.  Or you can, if you can swing a soldering iron.

There was also an EL-34 based SE-85 and a 300b amp.  Pretty rare, I have seen these but never heard them playing.

The SEP-1SE preamp is also pretty good, after rebuilding.  I have never seen a "non" SE version.  All the ones I have seen were noisy until recapped and bad diodes were replaced.  There was an optional phono board and if your SEP-1SE has one great; just don't pay any more for it.  It's not all that good and you are better off with an external phono stage IMO.