My Dynaudio C1's (85db) is being driven by an Octave V70SE (70w @ 4ohms). The Octave beats the snot out my Bryston B100 rated at 180w @4ohm. I can't explain it but the tubes to me have more control and power and can be drive those C1's as far as the Bryston did.
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Recently I read somewhere that tube amps are not good at driving speakers which have a wide impedance curve, say if the impedance swings from 3-20 Ohms across the audio range.Not many speakers have such wild impedance curves. The SoundLab speakers (which I own) do have such a rise in the lower freqencies. This ultimately puts less stress on the amp in the area that often causes the amp the most challenge.
Well designed tube amps do a better job to provide their rated power across a wider load impedance range. I am sure Atmasphere will chime in here to cover the technical details.
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:
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.