How to shop for speakers?

Sensitivity won't matter, if you have inductive speakers and enough power to get to the listening levels that you require. I looked up the Mac MC275, it is 75 watts, you should be fine with a minimum of 86db sensitivity. That will give you 101db output @ 32 watts. Not rock concert levels, but certainly live Jazz venue volume levels.

HiFiTime & HiFiHvn, most tube amps (that have output transformers) are designed to provide approximately the same maximum amount of power when a 4 ohm speaker is connected to the 4 ohm tap, as when an 8 ohm speaker is connected to the 8 ohm tap. There are several things to watch out for, though:

1)If an 8 ohm speaker is connected to the 4 ohm tap, which is sometimes done to achieve tighter bass or other effects on sound quality, then there would be a 3db volume drop (compared to the 8 ohm speaker being connected to the 8 ohm tap).

2)If the speaker sensitivity is rated with respect to 2.83 volts, rather than with respect to 1 watt (2.83 volts corresponds to 1 watt into 8 ohms), and if the speaker's nominal impedance is 4 ohms, then a 90db/2.83V/1m sensitivity would be only 87db/1W/1m.

3)Fluctuations in the speaker's impedance vs. frequency curve will assume different significance for a 4 ohm speaker connected to the 4 ohm tap than for an 8 ohm speaker connected to the 8 ohm tap, if the fluctuations are not in the same proportion. For example, if the impedance of a nominally 4 ohm speaker varies from 2 ohms to 8 ohms over the frequency range, a greater impact on frequency response would result (with a tube amp) than in the case of an 8 ohm speaker whose actual impedance had the same tolerance (-2/+4 ohms, meaning impedance varying between 6 ohms and 12 ohms).

Along the lines of Ralph's comment the matchup of the speaker's variations of impedance as a function of frequency, and the amplifier's output impedance, should be consistent with the designer's intention.

Also, I second Tim's comments about the importance of the phase angle of the impedance, and its variations with frequency. A low impedance magnitude coupled with a highly capacitive phase angle is a worst case load.

Regards,
-- Al

Thanks guys.I guess I'll still keep using the 3 db for reference.Sometimes the 4ohm tap does sound better with some 8 ohm nominal rated speakers. That was just a guess.I had a couple of different Conrad Johnson amps,and from what I remember,they wired them all for 4 ohms out of their factory.They told me that they sound the best this way.I see an amp for sale now that has a 6 ohm,and 16 ohm output.Might be a compromise between the two.

Thanks Almarg and Timlub.Some odd things do happen sometimes.In the 80s I believe,a dealer friend tried two pairs of Rogers 15 ohm speakers on a tube amp,and it resulted in a volume drop that was enough to stop him from ordering a second pair for his home.This was on the 8 ohm tape.Being a tech,he was somewhat disappointed.He didn't bother going any further. It seems like we always are learning.I think there are some Magneplanar speakers that are a decent 4 ohm load.It would be nice to have Stereophile,or others do tests on a lot more speakers.That sure would help.To many speakers are rated 8 ohms that have characteristics that should put them in the 4 ohm category in my opinion.It makes it hard for people that starting home theater systems that can only handle 8 ohm loads.

If an 8 ohm speaker is connected to the 4 ohm tap, which is sometimes done to achieve tighter bass or other effects on sound quality, then there would be a 3db volume drop (compared to the 8 ohm speaker being connected to the 8 ohm tap).

Slight correction to this statement in my earlier post. The statement should say:

If an 8 ohm speaker is connected to the 4 ohm tap, which is sometimes done to achieve tighter bass or other effects on sound quality, then there would be a volume drop of up to 3db, the exact value depending on the amplifier's output impedance (compared to the 8 ohm speaker being connected to the 8 ohm tap).

Best regards,
-- Al