By appropriate, I mean a speaker that is efficient enough to provide the volume you need in the room you are using, that has the high-ish, relatively flat impedance curve that tube amps work best with. For the reasons that Almarg and Atmasphere elucidated, that efficiency had better be pretty high (>95 DB or more) if you want to make reasonably high volumes in a moderate to large rooms using an SET amp, but there are a good number of push pulls tube amps, w or wo OPTs that can drive speakers in the 85+dB range if the impedance is flat and >= 6 ohms.
That "tube sound" and power ratings
This might be a newbie question since I've only begun researching tube technology. I understand to some degree the theory that tube sound is partly related to second harmonic distortion vs. the more prevalent odd order harmonic characteristics of SS. If "tubies" prefer that sound (I might be one of them), does it make sense to carefully match an amplifier's power rating such that it is NOT TOO HIGH for the speakers it's driving? If the rating is too high won't that mean lower distortion and hence less tube sound for a given volume for those speakers than a lower power tube amp (in general that is - I realize not all Watts are the same). So won't a high wattage tube amp have less of the special tube sound "tubies" like at their preferred listening volume?
I realize I'm likely missing something here. Set me straight!
I realize I'm likely missing something here. Set me straight!