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.

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