Have you guys (Andy, Beetle, Holco or Tom) found this problematic for your personal system power amps to drive? As I understand it, this is correlated in drops between 4 Ohms down to 2 Ohms across much of the speakers' frequencies. 3 Ohm nominal.
The impedance of a speaker can be a bit complicated (but it doesn't have to be). The other side is how the amp drives the speakers if the amp is a no-feedback or feedback design.
In general, the higher the impedance is the better but that statement is a bit overly simplified since higher at some frequencies may not be good. In most speakers, the impedance is lowest at low freq. around 100 - 200 hz and rises toward the higher freq. The low freq is where most amp will have issue if it stays too low. I think the impedance of 3ohm or above is probably OK, but below 3ohm, some low power amp probably will have problem. At high frequencies, low impedance is usually not an issue since high frequencies do not demand as much current vs. low freq.
As the freq. rises higher, you probably don't want impedance to get too high (due to inductance rising) which will result in treble harshness. In some speakers, there is an "impedance correction network" to reduce the rising impedance so that it won't be a problem for tube amplifiers. So this is where high impedance is not good either.
As for impedance angle, it is mostly negative (capacitive) at low freq. and gets more positive (inductance) at higher frequencies. As long as there is not excessive swing from negative to positive, it should be fine. In general, you don't want the phase angle excessive negative at low frequencies and excessively positive at high frequencies. Too negative angle at low freq. will be hard to amp to drive. Too positive at high frequencies may result in treble harshness. In general, capacitive load demands more current and reduces dynamic, whereas inductive load causes high frequencies issue.
The other side is the amp design. Some amp are design with no feedback and some with feedback. Most people agree that no feedback is better sounding vs. feedback. But amp with no feedback are more sensitive to speaker impedance swing so it is a trade off. Interestingly enough, most low cost amplifiers are designed with relatively high feedback because it costs a lot more to design a good amp with no feedback. Tube amps tend to be most sensitive to impedance since most of them are designed with very little feedback or no feedback at all and exacerbated because tubes usually don't have a lot of drive current.
For those who are interested in feedback vs. no feedback argument, there is a good article from PASS lab that I included the link below:
http://www.firstwatt.com/pdf/art_dist_fdbk.pdf 
