Minimum recommended amplification requirements and amplifier matching

For years I drove Proac Response 2.5's with a Cary CAD-301, a rare SE triode 300B beast putting out a max rated 14W.  See the Stereophile impedance plot of the 2.5s, definitely an 8ohm speaker.  Definitely plenty loud enough when required.  Reinforces my belief that not all watts are the same.

The volume (power) level and the impedance of the speaker are different. For a given impedance regardless of the power, if the impedance is raised the distortion will be lower. So ideally the speaker would be of the efficiency and impedance that you can take advantage of both.

Very appreciative of the clarification @atmasphere 

One last question on this if I may. You mentioned in your 12:16pm post "...all other things being equal". Am I correct that you are also speaking of the *sound pressure level*? I ask because if so, again it would seem to me that higher speakers impedance (say 16 ohm instead of 8) would mean that one would need to raise the volume/power to achieve the same SPL, and hence in so doing could increase distortion if the amp is not operating in its "comfort zone".

Thanks again, atmasphere.

You mentioned in your 12:16pm post "...all other things being equal". Am I correct that you are also speaking of the *sound pressure level*? I ask because if so, again it would seem to me that higher speakers impedance (say 16 ohm instead of 8) would mean that one would need to raise the volume/power to achieve the same SPL, and hence in so doing could increase distortion if the amp is not operating in its "comfort zone".

The 'all other things being equal' is meant to suggest that if you could magically have the same exact speakers, differing only in impedance.

16 ohm speakers do not need more power. Impedance and efficiency are two different issues; a watt into 4 ohms is the same sound pressure as a watt into 16 ohms 'all other things being equal'.

If you are used to the sensitivity spec rather than efficiency, then the 16 ohms speakers might seem harder to drive. This is because the sensitivity spec is based on voltage (2.83 volts) rather than power (1 watt). Into 8 ohms the two are the same; into 4 ohms the power is 2 watts if the voltage is the same, and into 16 ohms the power is 1/2 watt if the voltage is the same.  This can make the higher impedance speaker appear harder to drive, but if we are talking about tube amps, they are no different; in fact quite often a tube amp will make more power into 16 ohms rather than less.

Thank you
@atmasphere

I asked this question in another discussion but this one suits better.

If a speaker manufacturer has the following specs; do they mean the recommended power in 8 ohms or 6 ohms?

Specs:
Impedance: 6 ohms
Recommended power: Minimum 50wpc