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.