4 ohms , 6 ohms, or 8 ohms???

Almost all speakers have varying impedances, and there is no 1 right way to judge what a speaker's "nominal" impedance is.  Some by average, some by minimum. There's a great deal of spec doctoring or wishful thinking involved.

You want to pick a tap that is closest to the measured values, so for this speaker 4 is it. However, it would be worth listening to other taps.

The issue isn't the impedance peak, but the minimums. I wouldn't call this a 4 Ohm speaker, I'd call it a 3 Ohm speaker. It's really abnormally, and broadly low in impedance to call it anything more.  That most of the bass is below this range means the amp is going to be working hardest where the music amplitude will be greatest.

Interestingly, Stereophile reviewed these speakers with a McIntosh with 2 Ohm taps, that seems much more appropriate than 4 Ohms. Also interesting was the change going to the Bel Canto's which don't have very good low impedance drive.

You want something pretty stiff. Less expensive but very good alternatives in the SS land would be the Parasound A21 stereo amps. That or better is where I would go.

Honestly I don't like the impedance curve at all, but if you like the sound you are going to have to carefully match them to the right amplifier to get the most out of them.

I believe, Ralph of Atma-Sphere, recommends using a Zero device for low impedance speakers.Here is a link
 http://www.atma-sphere.com/Support#FAQ
On the right hand side, he discusses what you are asking about. The Zero link is highlighted in blue.

One other thought. If they are bi-wirable, consider passive bi-amping. Get a solid state amp for the bass section, and keep your tubes up top.

Best,

Erik

The thing about a set of ZEROs is that they can allow you more flexibility.

For example you could run the amp straight into the mid/tweeter section while using the ZEROs for the bass section (assuming that the speaker is bi-wirable).

OTOH you're not going to hurt the amp trying different taps so I would do so. But it does seem as if the 4ohm tap is likely the most appropriate. At any rate, keep your speaker cables short (under 3 feet is a good idea) and make sure the speaker connections are tight.