Tube amps and speaker matching

Tubes are expensive power. With efficiencies less than 89 db or so, you start to need a lot of power to really make the speaker play, although there is a lot of variance due to rooms and taste. So the power rule of thumb thing is generally OK but should not be regarded as cast in concrete.

As far as impedance swings go, that all has to do with the type of speaker and the intention of the designer. For example Sound Labs and older Quads have wide impedance swings but tubes do quite well with them.

With regards to intention of the designer see:
http://www.atma-sphere.com/papers/paradigm_paper2.html

As for general guideline/rule of thumb things I can add another one:

"If you are investing in a tube amplifier, your tube amplifier dollar will be best served by a speaker that is at least 8 ohms or more, all other things being equal."

Of course 'all other things' are *never* equal, what this comment is speaking to is that all tube amplifiers whether they have a 4 ohm tap or not will have more power (although not by all that much) lower distortion and possibly wider bandwidth when driving higher impedances.

Bottom line is YMMV, so audition is always the best policy!

My 22 watt SET drove the Acoustic Zen Adagio speakers I was using at the time, with better grip and control than 180watt Karan K180 integrated. The Adagios from memory, were 89db sensitivity and dropped to a minimum of 6ohm impedence. So, not all watts are created equal, but I would agree that tubes do'nt match well with low impedence. I would say 6ohms ideally, as a theoretical minimum.
As always though, you get suprisingly effective matches which should'nt work and visa versa.
The moral, try before you buy.

Sensitivity is only so important. It is also important how stable the impedance curve is. For Example:

An 8 ohm 91db speaker that has impedance swing from 2-8 ohms will not be controlled as well with medium powered tube amps as an 8 ohm 87db speaker that has a minimum impedance of 7 ohms will.

Remember sensitivity ratings are Xdb with one watt at one meter away. This means for speakers in the mid to upper 80's it is very possible to power them with average power tube amps 40-80watts. That is if the impedance does not swing to much in the lower registers.

I am sorry I did not read your first paragraph of your question. So yes the swings are important. And just like someone said above, impedance swings that large are rarely seen. Atmosphere makes some excellent points. Sometimes you just have to try and see how it sounds!!

I appreciate the input from all, thank you very much.