Aren't 4 ohm loads more problematic for tube amps in general? Isn't that part of teh [sic] reason why you advocate 8 ohm or higher speakers for use with your tube amps?
This is true. The fact of the matter is that **all** amplifier technologies sound better (better= smoother, greater detail, often more authority, and in tubes less lower-ordered distortions that contribute to the 'tube sound') when driving higher impedances. IME 8 ohms is OK (better than 4) but where the 'magic' happens is with 16 ohms.
So I usually put it this way: If *sound quality* is your goal, 16 ohms will sound better than 4 or 8 ohms, all other things being equal. If **sound pressure** is your goal, then 4 ohms (or possibly less) is preferred if you have a transistor amp that can handle the load.
The point is that in high end audio, we are more concerned about getting it to sound real rather than anything else. Quite often (as in Unsound's case per his last post), things might be sounding *good enough* so that one is satisfied, but that is rarely the same as saying that the sound is as good as it could be. Its whether one is interested in making that step in the interest of better sound. I say this because quite often a speaker design could be a 16 ohm design or 8, rather than 4, without seriously altering the design. It does not have to be more expensive. Its really up to the speaker manufacturers, but I think most of them don't want to raise the impedance because they fear loss of 'voltage sensitivity'.
Have you noticed that there is speaker efficiency, 1 watt/1 meter, and there is also sensitivity, 2.83 volts/1 meter? The fact of the matter is 16 ohm speakers might be less sensitive, but they will not be less efficient. If this seems hard to get your brain around, consider the idea that the sensitivity spec was created to cover for the fact that many speakers are not as efficient as they are made out to be.