[Rclark]

 I guess I should be asking what the difference is, sonically between most low efficiency drivers and most high efficiency drivers.

 Because, if you can achieve superior sound with less watts, why would you want to have something that takes a ton of power to drive? What's the advantage of having low efficiency?

 The only thing I can think of is maximum spl, perhaps low efficiency drivers can handle a lot more power or something along those lines. I think that in terms of cone speakers only, because my planars are low efficiency, but are not spl monsters by any means.

 Or maybe low efficiency drivers can have better sound quality because of that massive electrical force being applied for control.

 

[JohnR]

If you're talking about woofers, there's a general trade-off that says you can have efficiency, small size, and low frequencies - pick any two I think this is referred to as Hoffman's Iron Law (no idea why).

It would probably be helpful if you pointed to something specific that you were looking at, if you're after more, um, specifics.

[sonicboom]

What John said is correct.  There's an impedance mismatch between the cone and the air at low frequencies that raises the excursion of the driver disproportionately the lower you go.  That is to say that the efficiency at the lower frequencies sets your systems efficiency. 

Depending on how low you want to go for a given size speaker, you will have to sacrifice the efficiency at the mid-band frequency range to accomplish your goal.  If we were to take the typical woofer/mid from SEAS, Scan-speak etc. and just increased the BL of the motor by installing a bigger magnet, we could raise the efficiency of the driver by a large margin but this would take place at the middle frequencies.  The bass would remain pretty much unchanged thus basically shifting  the drivers operating range higher.

This by the way is the case with high efficiency drivers when compared to their similarly sized hi-end counterparts.  As JohnR alluded to with Hoffman's Iron Law, you can't have your cake and eat too.