Is this enough power?

Phase angle is how much current leads or lags voltage.
In a pure resistive circuit they are in sync.

The wiki has some good illustrations.
http://en.wikipedia.org/wiki/Power_factor

For stereo / hi fi, this presentation is most useful
http://www.hometheaterhifi.com/component/content/article/228.html

The main thing to remember is that any time current and voltage are not perfectly in sync, less power is delivered to the load.

It is pretty much purely electrical. Maggies for example, if you simply look at impedance may be considered a difficult load. This is not necessarily true, since the phase angle is moderate thruout the frequency range. Other low sensitivity speakers which like tubes may be like the Harbeth.

Here is a good example of a fairly large phase angle coupled with a low impedance dip. I don't know if these guys work well with tubes, or not.

http://www.stereophile.com/content/focal-chorus-826w-30th-anniversary-edition-loudspeaker-measurements

The phase angle is not a single number or a set of numbers. The data will be in the form of a frequency response graph, but with + and - phase angle on the vertical axis. Stereophile prints such data on the same graph that plots impedance at various frequencies. What makes a speaker difficult to drive would be an extreme phase angle (e.g. -45 degrees) at a point where impedance is at its lowest point, and particularly if such combination is at a frequency with a lot of musical energy (e.g., 80-200 hz).

Fortunately, reports like that of Stereophile, will not only provide data but an interpretation of its meaning so you don't really have to know how to read the graphs.

My guess is that 30 watts may work, provided you don't play the system at really high levels. For reasons I don't quite understand, I find that distortion from a shortage of power will be most evident on works with a large chorus. At a subjective level that does not seem that loud, a work like Rachmaninov's Vespers may begin to sound distorted if your amp is not fully up to the task (maybe its because human hearing is most sensitive and tuned to the human voice). If you get a chance to try the amp, bring along such works.

One of the advantages of a good SET amp is that they are quite musically satisfying and sound right at lower volumes so you probably would not really need to push them that hard. I personally don't mind trading off the rare times when I really want to play the system at extreme volume in order to get the desirable characteristics of a good amp for 99.9999% of the time.

Good, question--but YOU, are the only person that can answer this question. It works for ME, on paper...but that doesn't mean you'll be satisfied.

Good listening,
Larry

Thanks again for the explanations. I finally found a graph with phase.

Looks like the lowest phase angle is approx. -40 degrees at 30Hz where the impedance is 10 ohms. Other than that dip, the phase angle is above 0 degrees.

From what I understand from the above posts, this means this should be an easier speaker to drive and that the 30w of SET should be enough.

Have also found that most tests show that the speaker is 90db/2.83V/1m. Maybe the manufacturere specs are conservative.

Looks like the lowest phase angle is approx. -40 degrees at 30Hz where the impedance is 10 ohms. Other than that dip, the phase angle is above 0 degrees.

From what I understand from the above posts, this means this should be an easier speaker to drive and that the 30w of SET should be enough.

Have also found that most tests show that the speaker is 90db/2.83V/1m. Maybe the manufacturere specs are conservative.

I would still suggest caution before reaching that conclusion. Given that the speaker's impedance is 4 ohms below 1kHz, its efficiency in that region (where most of the power is typically required) is only 87db/1W/1m, because 2.83 volts into 4 ohms corresponds to 2 watts.

Assuming that you are referring to dynamic (non-planar) speakers, it can be calculated that 30 watts into each of the two speakers corresponds to a 95db sound pressure level at a distance of about 10 feet. As I indicated earlier, that should be good enough for most recordings but not for all.

Regards,
-- Al