If an amp has 4 ohm taps

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.