Is 5 Watts enough for Horns ?

The guy I bought the deHavilland gear from had the faceplates of the mono amps & preamp goldplated at a plating shop. He had an oversized volume knob machined at a machine shop. The volume knob & source selector
knob are also gold plated

Now that's what anyone'd call a serious hobbyist!

Old Klipsch speakers had good quality Alnico drivers in them -- until the price of cobalt (or was it magnesium) shot up sky-hi.

The best amp I heard on the Avantgardes was a 1.56 watt 45 tube amp from Exemplar Audio. I think that a 300B SET would certainly be powerful enough. I presently have 93 db efficient speakers and 300B amps are not enough.

I have Trio 2.2 (8 ohms). I have used Viva Aurora (845 set)monos for three years. Mine has the transformer input. This is a must with horns, the tube input version is too noisy. Mine is dead quiet. Recently I tried a friends Wavac 300b. Enough power. The 845's are just totally more effortless though and better balanced (very extended at the extremes). I also have now a Yamamoto A-08s (45 set). It is ok sounding in comparison but not as good as the Viva. It is also enough power. With trios you have a speaker that is capable of effortless dynamics and presence. You will not get this from a 45 set. With the 45 set the trios aren't that much better sounding than dynamic speakers. I would go with a minimum of a 300b or maybe the Art Audio PX-25 I keep hearing so much about. My 845 IMO is much better than these and not just in power, all other areas too, very clean sounding. I have not heard the px-25 in my system though. 845 amps are very expensive to build and if one is chosen it must be of the best quality or I would chose something else. There are only a few manufacturers building to this quality. I have heard many comments saying the 845 tube is not an audio tube and can't compete with the lower powered tubes. Yes, for many lower priced 845 amps out there. But when it is built right, and top quality, IMO it is hard to compete with it.

I doubt if you are ever using over .5 watts on either the Duos or Trios. In my experience as well as in a test done at the old VSAC in Silverdale WA, the 45 tube in its old American made form is clearly the best sounding output tube, if your speakers are sufficiently efficient. The old 2A3 single plates are close.

There are many variations in amps using tube output stages and certainly power supplies, parts quality, and other design features can alter the impact of the output tubes.

I have no experience with the 845 tube, but I have found great differences among 211s. Again the old RCAs clearly outperform the new Chinese tubes. So in reality we are talking not only about comparing 45s, 300Bs, and 845 but also NOS versus newer production.

The simple answer to the original posting is that the 8 watts from a 300B SET is quite sufficient for 107 db efficient speakers in the normal listening room.