Is this enough power?

If the speakers are of moderate phase angle, you're good to go.

Large phase angles or large phase angles at impedance mins will tax the amp.

Good job getting the impedance data. sounds fine.
However, your job is not done yet! See if you can find some phase data. This IMO is the true test of 'good load/bad load'. Huge phase angles at low impedance will really drain all but the most robust amp. Speakers which are heavily capacitive do not like tubes....or perhaps the other way 'round.

Trying the various taps on the amp? Best idea yet.

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

Audio,
I had hoped you'd look at the links a little better.

One thing I didn't say and Larry didn't touch on either is that phase angle has 2 aspects. The general measure is 'reactance'. However, there are 2 kinds of reactance.
Capacitive and inductive reactance are both measurable but it is the capacitive aspect which will give tube amps a problem. I believe in the stereophile graphs, that is the part below the 'zero' degree line.

Look at the link i provided to the 'Smith Chart' and go to the 2nd page for the presentation I prefer. It is both impedance and reactance represented by a SINGLE line in a big curly q shape. The distance down the line is frequency so that is tougher to figure out. The tiny curly q's are crossover frequencies, usually