tube amps and electrostatics

Is it only flaky if the amp can't handle it? Perhaps the speakers performance benefits from it?

Stated differently, is it possible to design a cone speaker that performs well *AND* has benign impedance and phase angle curves? And is also reasonably sensitive??

yes! Roy Johnson of Green Mountain Audio has been doing this for a long time now. He's well known but not an audiophile household name like, say, B&W. But his speakers are all cone-driver types & they are often 6 Ohms & they have very little phase shift in the 200Hz-8KHz band (we're talking 10 degrees or less) & outside this band the phase shift is more but nowhere near the speakers that are members of the Society for the Cruetly to Amplifiers. Green Mountain Audio speakers are easy to drive - a 30W/ch RM10 from Roger Modjeski will drive many of his stand-mount speakers to sufficiently high SPL. The Green Mountain Audio speakers are in the 90dB sensitivity range.
here is a link to his Rio speaker design notes:
http://greenmountainaudio.com/storage/speakers/rio/Rio-Design-Concept.pdf
i realize that no phase plot is given but one can assume from the flatness of the impedance curve that the phase is also relatively flat (note that wild impedance curves & wild phase curves are related - when you see one, you see the other. Do an empirical check to convince yourself).
here's a review of the Callisto speakers on 6moons:
http://www.6moons.com/audioreviews/gma2/callisto.html

The Green Mountain Audio speakers always sound like music on all your genres of music. I recently heard them again at RMAF2013 & they once again delighted. Stereophile gave Green Mountain Audio some very good press:
http://www.stereophile.com/content/marigo-whirls-green-mountain
Roy Johnson on the left (& Marigo Audio Labs owner on Ron Hedrick on the right).

For me speakers from the company are really the best & I can't seem to listen to any other type of speaker as they seem "broken" to me (my understanding is that the Apogee Scintilla also has a 1st order x-over).
No financial or otherwise implied relationship with Green Mountain Audio. Just a happy former owner.
Thanks.

Thanks for the heads-up George. I'll do some checking. At this point, more from dumb luck, I may have a combo that works ok because my ARC Ref 150 has muscle power and a large reserve power supply, plus my speakers are reasonably sensitive (92 db). Otherwise, I would have been another hapless victim. I'll be smarter next time when I upgrade speakers. That's for sure.

Bifwynne, another speaker you might look at is Audiokinesis. His speakers are not only more efficient, they are also very easy loads (some of them are 16 ohms) without crazy phase angles, yet also quite musical and good bass extension.

I think you are correct that speaker manufacturers ought to be taken to task. Back in the old days when tubes were the only game in town, speakers were a lot more efficient and higher impedance. When the less expensive power of solid state came along, we saw both impedance and efficiency go down.

When transistors came along, amp manufactures realized they could build an amp with same power as a tube amp, but it might only cost 1/10th as much, yet they could still charge 90% of the tube amp retail cost. A similar thing was going on with speakers- its costs about 1/10th as much to build a lower efficiency driver (as opposed to a high efficiency driver). To get back some of the perceived loss of efficiency, the impedances headed south. IOW, its all about the dollars.

IMO its telling that you can have two speakers, one 10X more efficient, and the more efficient one can have the same bandwidth and be just as revealing, IOW not giving up anything for being more efficient.

I can't think of a good reason for a high end audio loudspeaker to be less than 8 ohms. I've mentioned this plenty of times before- the lower the impedance, the higher the distortion, making any amplifier harsher and less detailed due to the types of distortion and our human hearing/perceptual rules. IOW if you want to make a speaker that seems smoother and more revealing, all you have to do is increase its impedance (all other things being equal- same box characteristics, same crossover points). It will not be that the speaker itself is all that different, but the sound of the amp driving it will be!

"ARC Ref 150 has muscle power and a large reserve power supply, plus my speakers are reasonably sensitive (92 db)"

That's a pretty solid combo on paper that should be capable of delivering the goods with the right tweaks.

No system is inherently good. Its what you do with it (the "smart" tweaks) that matter.