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.

When I used my STR-104 with my Torii Jr.  I settled in with my favorite 2.5v tubes in the Str-104 being a RCA 866a in the rear socket and a 3B22  or a 2A3 (2nd) in the front socket.  My favorite 5v was a 300b paired with a type 83 Mercury tube or 2 866A with adapter.  Go back through this thread 3-6 pages and I posted my experiences with different tubes.
Fyi - Al told me not to use 2-2A3or 2-300b tubes at the same time.  They may sound light and the tubes life would be very short due to driving them too hard. A good sounding great value 300b tube is a Linlai 300b.  $60-70 on ebay or Aliexpress. IMO a high quality expensive 300b  tube would not be appreciated in the STR like an amp or pre.

Just to add to this piece, I spoke with AL recently, and he put it this way:

"...if you are using the STR with a power amp, then putting 2A3 x 2 or 300B x 2 will be more stressful to the tubes. In case if you are using the STR with a pre-amp, buffer, DAC or something not demanding power, then it should be fine and nothing will go wrong."

I've tried one 300B at various times, but in general I have been so pleased with sound from two 872A tubes, that I tend to stay put. Not having kids or pets minimizes my risk.