Impedance or sensitivity?

Another point associated with Al's is that not all amps will drive reactive loads properly.
Some amps which are good into resistors will fall flat on the face when confronted with reactive loads....especially at lower impedance.

So, couple a 'powerful' (into resistors) amp with a hi-sensitivity speaker which really is a bad/reactive/lo-impedance load and there 'ya go......oooops.

For the OP example? Let's see some phase data.

Al nailed it dead on. Most solid state amps will not have a problem. Many tube amps have a 4 ohm tap and have zero problems driving a non capacitive load. If you can get a frequency and phase chart, you will see rises and drops in the phase angles, the rises are inductive, tube amps don't mind these, the drop in phase angles are capacitive loads. When you have a steep phase angle, especially in the bass frequencies, most tube amps, just can't handle that. Try to get a copy of the frequency and phase curves for both the Harbeths and the VA Mozart, your questions will be quickly answered.

The opposite of a capactive load is inductive, right? Don't tubes have some problems with one or the other of these reactive loads?

I like this presentation of the data. The Smith Chart has been around for a while but doesn't get much hi-fi attention because of what may be learning curve issues. I like it because it, on a single chart with a single line, summarizes a speakers electrical behavior.

http://www.hometheaterhifi.com/component/content/article/228.html

It gets more complicated than just impedance and sensitivity. Some speakers have complex power robbing crossover networks. Some of the larger Thiel speaker systems come to mind. As always, the best way to find out if your amp will work with a given speaker is to try it in your system.