Amplifier stability with very low impedance, high efficiency mid/tweeter section???

To understand this better you need to understand what happens when you have an ideal voltage followed by two impedances.  One of the amp, one of the speaker.

V ---> Z(output) --> Z(speaker)  or for simplicity in a DC world:

V--> Ro --> Rs

You need to understand a little about how the varying Z (or R) will affect the voltage at the speaker.

You also need to understand that Zo/Ro is almost never flat, so the amplifier's output impedance is also changing.  The inability of an amplifier to double it's output as impedance is halved is related to this.

So a 100W amp that can't double all the way down to 2 Ohms will also probably not be able to double at 1 Watt output.

Lol MillerCarbon.  

Erik, I've looked at a bunch of other stereophile measurements of amps.  They all fluctuate in impedance but but it's not by very much, at least most of them.  I guess you would need an amplifier with an output impedance of less the .2 ohms across the spectrum and a beefy power supply/enough output deviced to drive a 2 ohm load safely.  

...low impedance of 2 ohms.  Would this be a problem for most amps? 

It can be a problem. My rule is that if the loudspeaker gets into the 2 ohm range I would want to match that with a very robust amp that boasts about its low impedance performance.  

Erik, I've looked at a bunch of other stereophile measurements of amps.  They all fluctuate in impedance but but it's not by very much, at least most of them. 

Yeah, but that's wiht a simulated 4 Ohm speaker. :) Lower speaker impedance = more fluctuation.  I'm not saying it will be unbearable. Just explaining how a more robust amp with a lower output impedance would behave.

In some cases the interaction could be euphonic.

But take an ESL, which may be 4 Ohms in the low range and 1/3 of an Ohm at the top. The combination of rising output impedance and dropping speaker impedance cause the amp to no longer give a flat frequency response. You lose treble.

@erik_squires You might want to leave ESLs out of this since they are not 'voltage driven' loudspeakers, unlike most box speakers. Their efficiency is not mapped by their impedance curve. So you need the same amount of power at 10KHz as you do at 100Hz to obtain the same sound pressure- and these two frequencies will be very different impedances! If you put an amp that acts as a voltage source on a speaker like that it will tend to be bright and lack bass impact as a result. There are exceptions- for example the Sound Lab ESLs have controls and switches on the back to allow you to address this problem.

@poseidon1500 There is not an amplifier made that will be harmed by the low impedance of your midrange/tweeter array. But there is a different issue which you might want to take into account!

All amplifiers make greater distortion into lower impedances. In addition, the ear uses higher ordered harmonics to sense sound pressure and so is keenly sensitive to their presence either natural or generated by amplifier distortion. It interprets them as 'bright and harsh'.

To complicate matters the ear has its greatest sensitivity at higher frequencies: between about 4-7KHz. This sensitivity is often called the 'Fletcher-Munson curve'. Something to keep in mind is that if you have an amp that can double power as impedance is halved (IOW, acts as a voltage source), harmonics of the bass notes will be audible coming from the high frequency array. This will contribute to brightness. Of course this will always be the case, the problem is that with many amplifiers as frequency is increased, there is a certain point where the feedback falls off inside the audio band. This causes the amp to make more distortion and this is very likely at or near the frequencies your MTM array is operating. If you are planning a conventional solid state amp this really sounds to me like a recipe for brightness and harshness.

If it were me I would insist on auditioning the speaker and amplifier combo in your home!