I can run my Lowthers with 1 to 2 watts and blow you out of the room. Big horn speakers like Altec A7's with over 100db efficiency can run on 3/4 to 1 watt easily
"I do not know about now but back a while ago I finally figured out the fewer gain stages the cleaner the sound but how can you run a speaker with less than 5 watts?"
I know what you are saying, but there are other factors that effect how clean sounding the amp is. Also, if you need more power, you can go SS. My Pass amps had only 2 gain stages. They had similar sonic qualities to low power tube amps.
I run a 99 db/w system with a compression horn midrange with a 5 watt pushpull amp or a 6.5 watt parallel single ended 2a3 triode amp. Both are quite adequate for the job. The only time I feel the need for more power is when it is playing acapella choral music; for some reason, this type of music begins to sound a little more muddled at surprisingly lower volume than other kinds of music.
The best thing about horn systems and really good low powered amps is that the music is VERY satisfying at LOWER volume than other setups. I find that the better my system gets, I listen at lower and lower volume.
If you're not bass freak, than other part of sound is sweet. I had diy 6l6(in SET design triode only) based amp 4...5 wpc and was powering JBL 4312 vintage monitors that are 94dB efficient. the bass went down to 48Hz flat and to 41Hz with 5dB slope. I could get 97db/m SPL when turned volume all the way up. The volume control was initially passive but then I decided to use 12AT7 NOS Telefunken single duo-triode for both channels.
The total cost of SET amp and speakers were in $150 ballpark when I built it 15 years ago.
Viridian disagree with the large horns needing large space to sound right this is wrong from my findings. If properly set up a large horn doesn't require any more room than other designs just maybe a bit more floor space and even that's debatable since most horns can be used in corner or by wall unlike monitors that require stands and need to be away from room boundaries these designs actually require more floor space than a large horn. I've used quite a few massive horns as computer speakers in near field office system.
I like a bit of power available, even on horns. I use triodes exclusively and my amps have only one stage of gain. I don't use all the power that my amps have, but since their distortion drops to very low levels (unmeasurable) at normal listening levels, the result is a very relaxed presentation with lots of authority and detail.
If you are running SETs, the key to real success is to make sure you are running a speaker that has enough efficiency such that the amp never makes more than about 20% of full power. In that way the distortion will also fall to unmeasurable levels and that's when that great inner detail emerges. With most SETs, to do this means that horns are pretty much the only game in town.
I've had good success getting them to work in smaller rooms no worries.