Speaker efficiency is sensitivity + impedance vs phase angles.
It is not solely the sensitivity spec, which is why you can’t assume a 90dB-sensitive speaker will mate well with a moderate/low power tube amp. This is why you don’t often see Wilson or Magico owners using tube power.
Low impedance drivers generally perform better than their high impedance counterparts. They require more from the amplifier, but if the amp is up to the task, the speakers will perform better and have greater dynamic range. This is why the large ultra-high-end speakers in the $30K^ range almost always have a low average impedance. The only exceptions might be those with unconventional bass loading and compression/horn drivers. This isn’t to say all speakers with a low nominal impedance are “high-end.” Many low-nominal speakers have 8-ohm drivers, but the combination and number of drivers coupled with complex crossovers lowers the speaker’s average impedance. This is why it’s not uncommon for a <$4K tower speaker to have a 4 or 6 ohm nominal rating when in fact all its drivers (excluding the tweeters) are 8-ohm.
All but very few speakers have impedance swings that will result in non-linearity when coupled with tube amps. The exceptions are rare. One reason tube amps are so popular is because the typical speaker load will result in a boost of the midrange and upper bass frequencies. It makes them louder and thus often results in the false perception of greater resolution and power delivery. It’s why you’ll encounter comments on the forums from Joe Schmo who claims his 20 WPC tube amp sounds more powerful than his 100 WPC solid state amp. The reality is 20 watts is 20 watts, and Joe just hasn’t cranked the volume quite enough for the compression to become obvious. And since tube amps “soft clip,” when overdriven, the sound flattens before it becomes obviously distorted.