Welcome to Team STR JHC! If you're using 2A3s and 866As I'm guessing you've got a STR-104. I agree you ought to try the STR in both the amp and preamp, but you'll need to use the 866As with the UFO, the 2A3s don't have enough juice. I use my STR-1002 with my (non-Decware) pre/headphone amp and couldn't be happier. You've probably already read through this thread but if you haven't I encourage you to do so, there is a lot of interesting experience here.

FWIW the 2A3 is a power triode, similar to the the 300B but lower power. It has a 2.5V filament, there is also a 6A3 tube that is the same except the filament is 6V. They aren't common, but usually cost less than 2A3s and will work with the STR set to 5V if you are interested.

Please keep us updated as you get more experience with the STR in your system.

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I have indoor cats and (so far) have been unwilling to try any of the mercury vapor tubes so I don't have any direct experience. But it seems like everyone who tries them really likes them.

My system includes two UFO25s and I've toyed with the idea of getting a pair of 104s for them so I can use more of my collection of rectifiers. But that isn't happening any time soon, if ever.