If an amp has 4 ohm taps

So I would think that generally speaking, if one had an amp with only 8 ohm taps (which is probably includes quite a few amps) one shouldn’t even consider speakers with 4 ohms listed as nominal impedance.

I’ll let the more knowledgeable and experienced answer most of what your questions are, but regarding this, the manufacturer would know the answer to what the capability of his amp is.

With the Coincident Frankenstein stereo 300B, I noticed it had 8 ohm taps only, so I asked the designer/owner if a 4 ohm speaker is fine, and the answer was that the 8 ohm taps are fine for any speaker from 4 to 12 ohms.

And this is what the manufacturer of my 45 amp states, but he is Japanese so read through the translation:

"An 8-ohm terminal has come out of 1 set of speaker terminal of this machine.
However, the speaker from 4ohms to 16ohms can be used.
Since it will lead to a sound quality fall if a middle terminal is prepared in an output transformer, it is being dared to make the output terminal into 1 set.
Although a little maximum output falls by speakers other than 8 ohms, a sound has little change and it can be satisfactorily used for it."

I can confirm both amps sound fantastic with my 4 ohm speakers; no idea if the tubes will last longer because of the "reduced maximum output" or if other factors are in place. I have heard my speakers are a flat and "easy" 4 ohm load, but I can't confirm. 

FWIW a 'nominal impedance load' of 4 ohms doesn't preclude the load from going lower to say 2 ohms or less at some frequencies which would require a high current amp. Most folks I think stay away from tube amps for just that reason, i.e. they don't drive low impedance loads well. Using the 4 ohm taps doesn't remedy this much if at all. Best to ID your speakers actual impedance 'curve' before you try to match your amp and speakers. Asks the manufacturer (?) or look for reviews which might have identified it.

 they don't drive low impedance loads well. Using the 4 ohm taps doesn't remedy this much if at all.

Thanks @newbee , I guess that was basically my question.

 a 'nominal impedance load' of 4 ohms doesn't preclude the load from going lower

Right.  The 4 ohm speaker I was kind of sort of looking into was the Totem Twin Tower Bison which has a listed sensitivity of 90 dB and the best I've been able to do so far on finding out how low it actually does go has been an AI answer of maybe 2.8 ohms.  

Impedance Curve

The link above is the only impedance curve I can find for those speakers (scroll down).

It looks like it doesn't go much below 3 ohms but it definitely not a flat curve.

My Magnepan LRS+ are 83 dB but have a relatively flat impedance curve around 3 ohms. They sound magical with my Muzishare X7 tube amp on the 4 ohm taps.

I can confirm both amps sound fantastic with my 4 ohm speakers; no idea if the tubes will last longer because of the "reduced maximum output" or if other factors are in place.

@mclinnguy There is. Its the load on the power tubes, which is transformed by the transformer to the speaker impedance. So if you cut the speaker load to 1/2 by putting a 4 Ohm load on an 8 Ohm tap, the result is the power tubes see a load that is 1.2 of what they are designed for.

This will cause increased distortion, lower power and shorter tube life. Might sound fine but it would sound better if the load on the transformer were correct.

@immatthewj If the best sound quality out of a tube amp is your goal then it is best to avoid lower impedance speakers. This is particularly true if the amp uses no feedback.

The lower impedance makes the speaker cables far more critical. They should be heavier gauge and also shorter for best results. Output transformers with a 4 Ohm tap tend to be less efficient than when they are driving 8 or 16 Ohm on the correct tap; the use of 4 Ohms can easily reduce the low frequency bandwidth by an octave. The transformer can often run warmer too and all that heat is amplifier power warming it up due to the (slightly) reduced efficiency.

All amplifiers make more distortion driving lower impedances. For this reason alone IMO 4 Ohms is a Bad Idea since keeping distortion down helps with reproducing low level detail and the like.

@immatthewj If the best sound quality out of a tube amp is your goal then it is best to avoid lower impedance speakers. This is particularly true if the amp uses no feedback.

The lower impedance makes the speaker cables far more critical. They should be heavier gauge and also shorter for best results. Output transformers with a 4 Ohm tap tend to be less efficient than when they are driving 8 or 16 Ohm on the correct tap; the use of 4 Ohms can easily reduce the low frequency bandwidth by an octave. The transformer can often run warmer too and all that heat is amplifier power warming it up due to the (slightly) reduced efficiency.

All amplifiers make more distortion driving lower impedances. For this reason alone IMO 4 Ohms is a Bad Idea since keeping distortion down helps with reproducing low level detail and the like.

Thanks for that explanation, @atmasphere . I will leave the 4 ohm Totem Bisons (Twin Towers) that I was kind of sort of looking at alone.  And that also includes the 4 ohm Totem Arro's that I found on ebay for what I was considering a price that I could afford to experiment with.

Thank you for the link, @newfzx7 .

The impedance curve at the link provided indicates the lowest it goes is approximately 3.5-ohms. Using the either the 4 or 8 ohm taps will be fine. Plug your speakers into each set of taps one at a time and see which ones sound better. 

I've had tube amps with 4 and 8 ohm taps, irrespective of what speaker I had or it's relative impedance I never found an amp that sounded better using the 4 ohm taps, over the 8's.

For those way more knowledgeable than me, where should I connect my KLH Model 5 speakers to my McIntosh 225 amp?  The 4 ohms tap?

THE AMP: Stereo or Twin Amp: (Center tap internally grounded) 600 ohms (One side internally grounded) 4 ohms, 8 ohms, 16 ohms, 150 ohms, 200 ohms

THE SPEAKERS: KLH Model Five speaker has a nominal impedance rating of 6 ohms with a minimum impedance of around 3.5 ohms at 140Hz

Speakers are a complex load with resistive, inductive and capacitive components, as well as a back EMF generated by the speaker and damped by the amps out stage. More sensitive speakers have greater back EMF. All that rolls up into a goat rodeo called complex impedance. Serial inductance causes current and voltage to become out of phase, so when that happens, impedance drops - current draw increases as voltage drops. That’s the definition of a low impedance. For reasons I don’t fully understand most modern loudspeakers in terms of current requirements (certain LS/35a models notwithstanding) behave more like a 4 Ohm - or even lower - load.

Vacuum tube amps use output transformers to convert voltage to current. He 4,8, and 16 Ohm taps represent the transformer designers best notion of how to best match the speaker impedance load. That’s an oversimplification,, of course, there’s much more to the art, science, and craft of transformers, but impedance matching is the heart of it. And that’s why it’s difficult to generalize about things like which tap to use with what speaker. Best to try your options. You won’t hurt anything, but it may not be ooptimal choice.