What is the benefit of low efficiency speakers?

Upper, the easy answer to your revised question is that there's no easy answer and there are LOTS of variables.

1. Designs that use multiple identical drivers (such as the Epiphanys) wire them in parallel or a combination of series and parallel to get a high-enough impedance so that the system is driveable. I BELIEVE* the best sounding method of wiring is parallel, but that produces the lowest impedance, and that result may be too low to drive. My Kindel PLS-As, designed in the mid-'80s, have 16 1"-dome tweeters of 16 ohms impedance wired in parallel. Including a resistive network, the PLS-A's impedance dips to about 2 ohms in the trebles.

2. A designer's favorite driver may be available in a 'wrong' impedance. Eg, a designer may want to use woofer A and tweeter C, each of which has a nominal impedance of 8 ohms, but the MR driver he/she wants to use happens to be a 4-ohm unit. If it's sensitive enough, a series resistor could be added, but if it's not, I don't know what the designer could do...beside pick a different driver

Some of us surely have lots more knowledge of this, and maybe lots of us will learn something from your questions.

* If someone KNOWS different, pls correct me.
.

The comments made by a couple of people talking about how speaker designers moved to lower resistance/less efficient speakers in the quest for more SPL and more bass is my recollection, too. That also helped to start the monster amp movement in the late 70s which peaked in the late 80s; at that time all we seemed to care about was the amount of capacitance a solid state amp had. Tube amps were weird back then to most of us. Check out the older ADS speaker designs from the late 70s and early 80s and that would be the poster speaker for the movement to less efficient designs in the quest for moderate sized speakers with lots of bass. This is also the reason so many mini moniters require LOTS of power to drive them and make bass.

This is a quote from Roger Russell's website on the change McIntosh made to its speakers in 1993:

Changes were made in 1993

Impedance of Home Systems Was Changed To 4 Ohms

All systems manufactured prior to 1993 were 8 ohms and had an industry standard sensitivity rating of 1 watt into 8 ohms. All systems made after this date were 4 ohms and have a rating of 2.83 volts, which is actually 2 watts into 4 ohms, making them appear to have greater sensitivity. To convert to a 1-watt level, subtract 3 dB from the rated sensitivity. For example: the LS350 is rated at 89dB for 2.83 volts. This converts to the industry standard rating of 86dB/watt/meter.

In addition, the new 4-ohm impedance requires the use of heavier speaker wire to maintain the same low losses compared to the earlier 8-ohm speakers. The total DC resistance of the wire must now be less than 0.4 ohms instead of 0.8 ohms.

Uppermidfi,

There's no benefit, in and of itself, in the lower impedance.

However, you have to remember that speaker design, like the
engineering of anything - is a series of trade-offs and
compromises.

The designer may trade-off a low impedance for some other
benefit.

My speakers are Apogee ribbons - notorious low impedance
amplifier killers.

Speaker drivers are motors - the force is proportional to
the magnetic field strength, the current, and the number
of turns in the voice coil.

A ribbon inherently is a single turn. Apogee used some of
the most powerful magnets available. The only way to get
the required force from a single turn driver was to up the
current - requiring low impedance.

These speakers are more difficult to drive than the average
speaker. It limits my choice of amplifier - a single-ended
triode tube amp is out of the question.

However, the speakers have some real advantages - and for
that I'm willing to pay the price of driving them with a
big beefy solid state amp.

Physics frequently excludes getting everything one wants in
a speaker - high efficiency, low bass, fine imaging...

Putting together a stereo system is a series of compromises
not unlike those faced by the engineers that design the
individual components of same.

I wouldn't put too much stake in what the impedance is.
Select your speakers, and then select an amp that can handle
the load they present.

Dr. Gregory Greenman
Physicist