Impedance or sensitivity?

Depends on the amp. Please tell us if the amp you want will be tube or SS.
Tube amps usually don't like low impedance speakers especially OTL amps. Impedance is hard, if not impossible, for tube amps to deal with when it gets lower. Tube amps can deal with low sensitivity by offering more power. It is costly, but not a limitation in many cases.

So in general impedance is harder for an amp to deal with. Solid state amps don't like high impedance. For example Soundlab speakers will have bass impedance in the 40's causing trouble and possible osillation for SS amps. They put out far fewer watts into high impedance loads - a small fraction of their rating.

It's worth considering too that the impedance spec is nominal. Impedance can dip below it and rise above it at various frquencies, and can have varying combinations of resistance, capacitance, and inductance. So two speakers with the same impedance spec can present an easy and difficult load respectively.

josh358 is right. Just because the stated speaker impedance is listed at 4 ohms does not make it harder to drive then all 8 ohm speakers. The important thing to know is how much and widely the impedance varies. A 4 ohm speaker that never dips below 4 ohms may well be easier to drive then an 8 ohm speaker that dips down to 2 ohms in the highs. Great point!

If the speaker has been reviewed in Stereophile, John Atkinson's measurements at the end of the review will usually provide useful commentary on how easy or difficult the speaker is to drive. That commentary will be based on the plot that is presented of the magnitude and phase angle of the speaker's impedance as a function of frequency, as well as its sensitivity. You can search for a review of a particular speaker at their site or via Google.

The most severe combination of circumstances would be a speaker that has low sensitivity, an impedance magnitude that drops to low values in the bass region (where lots of energy is usually required), and an impedance phase angle that is highly capacitive (i.e., that has a large negative value) at frequencies where the impedance magnitude is low.

Another point to be careful about is whether the speaker's sensitivity is defined with respect to 1 watt, or with respect to 2.83 volts. For an 8 ohm speaker (that is truly 8 ohms) it makes no difference, as 2.83 volts into 8 ohms corresponds to 1 watt. However, 2.83 volts into 4 ohms corresponds to 2 watts, so a 4 ohm speaker that produces 90db in response a 2.83V input produces only 87db in response to 1 watt. 2.83 volts into 6 ohms corresponds to 1.33 watts, which means that a 6 ohm 90db/2.83V speaker produces about 88.75db in response to 1 watt.

At the same time, keep in mind that the power capability of a solid state amp will increase essentially in proportion to a decrease in load impedance, up to some limit. A tube amp, on the other hand, will typically have a much smaller increase in power capability as load impedance decreases, while exhibiting a smaller decrease in power capability as load impedance increases. Those differences between the two kinds of amplifiers can have important implications with respect to tonal balance, as well as with respect to drive difficulty. See Atmasphere's paper on that subject.

Regards,
-- Al

In my opinion the impedance curve is more important. We had an opportunity to exhibit at THE SHOW in Las Vegas with FritzSpeakers. The Carbon 7's are 87dB sensitive. One would think this would not be a good match for a 300B zero feedback SET amp rated at 10 watts. However, the Carbon 7's are 6 ohm minimum and a very smooth impedance curve that matched up with the 6 ohm output transformers very nicely. We were getting very loud levels with no distortion from the amp. Several people were amazed at the fact that the combo could actually work let alone sound as good as it did.

Typical, but another excellent post by Al(Almarg).

So often, the published literature of a loudspeaker does not offer much useful insight, but the plot that comes with JA's Stereophile measurements more than justifies a subscription to the magazine.

I agree with Clio09 as a moderate to high impedance level 'with' a smmoth/relatively flat curve is very favorable to low power tubes.Sensitivity determines ultimate sound pressure levels but in terms of ease of driving a speaker its impedance.

My speaker has a 14 ohm load(10 watt minimum) and my 8 watt 300b amp just seems effortless driving them with any genre of music. Clio09`s experience with the Fritz speaker makes sense to me.

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.