Sensitivity is only so important. It is also important how stable the impedance curve is. For Example:
An 8 ohm 91db speaker that has impedance swing from 2-8 ohms will not be controlled as well with medium powered tube amps as an 8 ohm 87db speaker that has a minimum impedance of 7 ohms will.
Remember sensitivity ratings are Xdb with one watt at one meter away. This means for speakers in the mid to upper 80's it is very possible to power them with average power tube amps 40-80watts. That is if the impedance does not swing to much in the lower registers.
An 8 ohm 91db speaker that has impedance swing from 2-8 ohms will not be controlled as well with medium powered tube amps as an 8 ohm 87db speaker that has a minimum impedance of 7 ohms will.
Remember sensitivity ratings are Xdb with one watt at one meter away. This means for speakers in the mid to upper 80's it is very possible to power them with average power tube amps 40-80watts. That is if the impedance does not swing to much in the lower registers.