Preamp and impedance question

@roberttdid 
You bring up a good point.  I know the X5 use a 12” Eminence driver.  I look up their web site, and found the model and curve below.  It seems it dips down to 7 ohm around 200Hz.  So, only 1ohm lower than the nominal 8ohm.  In this case, I guess having only an 8ohm tap is ok?

And for the power subwoofer on the speaker, I guess the output impedance of the amp is irrelevant, right?

https://www.eminence.com/speakers/speaker-detail/?model=KL3012CX_8

If it only dips to 7ohm, then it will be just fine but the speaker is only part of the equation. The crossover will have an influence too. I don't know what they speaker does at the frequencies for the subwoofer. There will be some influence from the circuit for the mid. I expect there is some loading at low frequencies, but would be highly unlikely to be much less than 8 ohms.

@gte357s
No, what I meant was that if you have both 8-ohms and 4-ohms on the secondary, using 4-ohms will not be a problem as long as you don’t expect the maximum power the amp is capable of. The secondary will draw more current and if the xfmr and the tubes can handle it, then the amp will perform to spec.

If you are specifying a new transformer, then the above is moot. What you do is look at the impedance graph of your speaker for the entire audio range and look for the average impedance.

For example, my Thiel 3.6 speakers are rated at 4 ohms, however they are at 3 ohms from 100 hz to 10khz. If I were to spec a transformer, I would first see where the tube operates at the optimum power and THD. Say that the tube load is optimum at 3,000 ohms. I would then have the transformer wound to reflect a 3,000 ohm on the primary with a 3-ohm load on the secondary. Then I know what VA rating I need. With those specs in hand, I don’t need to worry about the transformer being able to drive the speaker because it is optimized for most of the impedance range. But in the lower frequencies when the impedance climbs to 8 ohms, the tube will be taken out of its power sweet spot but may still remain relatively low in THD. It's a tradeoff I'll take because it is, after all, a tube amp.

Thank you very much for all the inputs.  I need some time to digest the explanations.  I don’t understand all of them, but I feel this thread has the answers that I have been searching on the internet.  For now, I am good!  I will add only the volume pots, and not upgrade the transformer.  

I know the X5 use a 12” Eminence driver. I look up their web site, and found the model and curve below. It seems it dips down to 7 ohm

This is definitely not what it will be when it’s in the speaker, as xover will come into it also, plus the -phase angle what ever that might be.
And that if it say -50 degrees in the bass, which is not uncommon, combined that with the "5ohm" of what the manufacture "says" the speaker is, "could" represent a combined EPDR ("equivalent peak dissipation resistance") load even down to 2ohms depending on what the -phase angle is.
Be safe and get the 4ohm tap also, or you could end up with a thick ill defined bass, which will effect the lower mids also. 

To give you an idea the 12" Eminence driver originally in my ML Monolith III’s, was an 8ohm driver but in the Monolith’s they measured with the 3rd order xover a combined EPDR down to 2ohms presented to the amp below 100hz

Cheers George