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As Ralph Karsten (Atma-Sphere) and Roger Modjeski (Music Reference) will tell you, the lower the impedance tap on a tube amp, the lower the output impedance (higher damping factor) and distortion, generally speaking. Unfortunately, that comes with less power, except in the case of the unique Music Reference RM-200, which produces more power at lower impedances.
Modjeski recommends using the lowest impedance tap possible that provides the power required, as the lower impedance taps drive the output the tubes the least/softest (they will therefore last longer), and with the least distortion. Plus the lower output impedance is really a significant benefit, allowing the amp to produce the flattest frequency response it is capable of.
I'm in agreement with those who advocate trying all the available amplifier taps and listen. Each circumstance will be different and it's difficult to generalize if higher or lower tap is superior. My amplifier has 8 and 16 ohm taps and my speakers are 14 ohm nominal. I've tried both taps and the 16 ohm tap offers cleanly the better sound quality.
I believe that Ralph has pointed out in the past that using the higher ohm tap results in less distortion, increased bandwidth and better efficiency.
Roger Modjeski is definitely a proponent of the "light loading" approach. Honestly it depends on the particular amplifier and speaker involved. My SET amplifier has no NFB and this probably explains the better sound quality with the higher 16 ohm tap.
I believe that Ralph has pointed out in the past that using the higher ohm tap results in less distortion, increased bandwidth and better efficiency.@charles1dad Actually what I've said is that all amplifiers (tube or solid state) make less distortion into higher impedances; in the case of tube amps you also get wider bandwidth and greater efficiency.
Another way of putting this is if high quality audio reproduction is your goal, your amplifier dollar investment is best served by a loudspeaker of higher impedance- 8 ohms as opposed to 4 and 16 ohms as opposed to 8.
An additional benefit of 16 ohm loudspeakers is the speaker cable is far less critical for proper system sound whereas at 4 ohms its critical!