Whip, there's no necessary connection between speaker impedance and speaker efficiency; other design factors can make a lower impedance speaker either more or less efficient overall. For example, the M80 rated at 4 ohms is slightly more efficient than the M22 rated at 8 ohms and requires less power for a given sound output, so in that sense is easier to drive.
For a given voltage output from an amplifier, lower impedance allows greater current, following Ohm's Law(current equals voltage divided by resistance)and greater power(current times voltage). On the other hand, the factor which might increase the difficulty is that the higher current increases the heating effect on the amplifier, which could cause it to overheat and shut down.
A speaker or sub isn't "happier" if more maximum power is available if it doesn't need that power for the sound levels being used. Unused headroom is simply that: unused.
