Amplifier circuitry-4 ohm vs 8 ohm

Rogue Audio has told me in the past to try either 4 or 8 ohm and use whichever sounds best. 

Hmmm, I have two pair of Vienna Acoustics that are 6 ohm speakers and a simple EL34 amp with 8 ohm and 4 ohm taps. Thanks to the excellent posts above I am much better informed but more confused!  VA support told me to run off the 4 ohm tap, but the best advice seems to be try both and see which sounds better! 
 

Thanks for the science and advice. : )

The manufactures/distributors of these very very expensive products should be better positioned to answer these types of questions.

while above points help to clarify it, there’s still seems to be great confusion about relationship of voltage and its influence to amplifiers and speaker drivers.

It seems clear very few people understand these complex areas, and the communication of this understanding is very challenging. Kudos to those above who have been helpful.

Implied, but so far not stated specifically, is that the output tubes of an amplifier need to see a high load of several thousand ohms (the exact value depends on the tube and amp design.)  If you run an output tube straight into a speaker, whether 4 or 8 ohms, it looks like a dead short to the tube -- not good. 

And, straight from the tube, one is talking an output of up to hundreds of volts. Also not good for the speaker, which wants a lower voltage, but more amperage. 

We've been flying in the face of this for nearly 50 years making Output TransformerLess (OTL) amplifiers with a direct coupled output.

Somewhere along the way I realized that the distortion of the amplifier is also the 'sonic signature' that almost any amplifier has. So that meant if you could build a solid state amplifier with the same distortion spectra/signature, it would sound the same also. That proved out to be true.

So there are now solid state amps that allow you to leave the tube world behind without missing them for anything.

@atmasphere 

I've looked at many tube amplifiers. Interested to know which solid state amplifiers achieve the same goal as tubes?