That "tube sound" and power ratings

By appropriate, I mean a speaker that is efficient enough to provide the volume you need in the room you are using, that has the high-ish, relatively flat impedance curve that tube amps work best with. For the reasons that Almarg and Atmasphere elucidated, that efficiency had better be pretty high (>95 DB or more) if you want to make reasonably high volumes in a moderate to large rooms using an SET amp, but there are a good number of push pulls tube amps, w or wo OPTs that can drive speakers in the 85+dB range if the impedance is flat and >= 6 ohms.

Hazyi,
I can't speak for all owners of lower power tube amplifiers other than to say
I doubt they're intentionally seeking a "distorted" sound
experience. Al and Atmasphere's post are worth re reading. My speakers
are 94db at 14 ohms (minium 10 ohm load). My amplifier is an 8 watt SET
my average listening level is roughly 75-80db C weighted I sit 10 feet from
the speakers. Under these conditions I'm using only "fractions"
of 1 watt to achieve this very adequate listening volume. Most of my jazz
CDs have a dynamic range of 25-30db (some have an even wider range).
So there's ample head room in reserve when necessary to account for
musical peaks. The distortion levels are miniscule as I'm using mere
fractions of a single watt. The sound is pure and beautiful and there's no
need for "induced" distortion, In reality, quite the contrary. I
agree with Atmasphere's point regarding the advantages of high speaker
ohm impedance based on my daily listening experiences. Hazyi, I hope this
perspective helps make the point that gratuitous distortion isn't the appeal
for tube amplifiers.
Charles

Charles - thanks for another very helpful post! I assume you mean wrt average power output, at your listening level, you're only using fractions of a watt. I suppose it doesn't matter since even if it is much greater at peak maybe we're talking only 2-8W which you can handle within optimal range. I do realize you have a very efficient setup so my math may be off, but the "fractions of a watt" does sound a bit surprising and if it's true then I'm impressed - jealous even. These are the Coincidents that you refer to?

Yes I understand how power relates to impedance. No need to go there.

Do realize that yours is another data point and opinion, albeit a very popular one. Others commenting here have stated that over-driven tube amps might be appealing to some listeners. Actually they said it more strongly than that, but I'm trying to keep this civil.

We all know there are plenty of 8 ohm speakers. 16 ohms is a little harder to find. Some examples:

Classic Audio Loudspeakers
Audiokinesis (several models)
High Emotion Audio (Bella Twin- one of the best monitors I've heard)
Quad ESL57 and ESL 63
Lowther, PHY and other full-range drivers
Coincident Technology (not always 16 ohms, but definitely higher impedance)
Rogers LS35A
many vintage horns- Altec, JBL, EV, etc.
ZU Audio has made 32 ohm versions of some of their speakers
Some Pipedreams are set up as higher impedance

Its not important in a 16 ohm load that the impedance curve be ruler flat. In fact that is true of 8-ohm speakers as well. What does seem to be important is that the impedance variation does not fall below about 1/2 the nominal impedance, and when so doing is at a crossover point. ESLs seem to be an exception.

An example of a speaker that is difficult for a tube amplifier is the B&W 802D. This speaker is rated nominally 8 ohms but employs dual woofers, which are both 8 ohms and in parallel, so really this speaker should be rated at 4 ohms since the most power made by the amp will be seeing this load. If you see dual woofers in a loudspeaker, it is really good idea to inquire with the manufacturer about the impedance at the woofer frequency! If it is 4 ohms I would ask if its possible to make a version where the woofers are in series rather than in parallel. If you get a lot of pushback on this topic, move on!

Something that is really important when matching a tube amplifier to a loudspeaker is to understand that it can't double power as impedance is cut in half. Where this gets you in trouble with speaker designers is that many of them use the sensitivity specification when rating their speakers. Sensitivity is 2.83 volts into the speaker and measured at 1 meter. If the speaker is 8 ohms, this is one watt. But if the speaker is 4 ohms, this is 2 watts- IOW the speaker is actually 3 db less **efficient** than it would seem to appear.

Where the trouble spot is: many designers don't really see a difference between sensitivity and efficiency- so they think that by paralleling a driver, the efficiency of the speaker is increased by 3 db! IME when you try to correct them on this issue, often you get an angry response.

Now it happens that if you parallel a pair of woofers or put them in series, they will act exactly the same in the cabinet designed for them, the only difference is the values used in the crossover. I am going to use Kirchoff's Law here to make the point- if you have a single woofer and put one watt into it, it might make 92 db at one meter. If you have two of the same woofers in either parallel or in series and put 1 watt into the array, the result will be the each driver will have 1/2 watt on them- and the output will be exactly the same 92 db. If the efficiency were to actually go up, it would violate Kirchoff's Law, and because that is impossible (just like its impossible to violate Ohm's Law) it doesn't happen.

I can't tell you how hard it can be to get this fact across to speaker designers- its enough of a problem that I consider it an embarrassment (due to lack of engineering knowledge) when I encounter it. When you run into this, its guaranteed that the designer is using a transistor amplifier for design and testing and really does not understand the relationship between the amp and the speaker.

Now if you know all this above, and you know that higher impedances are good for tube amps as they will make less distortion and more power, then you are well ahead of many speaker designers and you now have a better chance of extracting all the performance out of the tube amp that otherwise is left on the table. Its your dollars in that amp- why not use it to best effect??