I think the non-obvious thing is that limits on amplifier output voltage tend to appear at the higher end of the power range, which is why you tend to see lower impedence speakers hanging out in the same space.

As an example, affordable car audio equipment used to be limited by the 13-14v you could get from the electrical system, so lower impedence speakers became the norm in order to get more power from the amp without having to use a DC-AC inverting power supply.

In home audio, I think you start hitting the limits of what the output devices (and the power supply filter caps) can handle without breaking down... you can go to higher voltages by specifying more expensive devices, but again going with a lower impedence for the speaker when possible is a very practical way to get more power.

Edited by bridgman (09/03/12 11:11 PM)