Preamp and impedance question

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

EPDR is not a meaningful term for this topic. That is a term more applicable to solid state amplifiers operated in the linear region and is most applicable to a discussion on thermal dissipation, not drive capability or optimization for frequency response and THD.

"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."

thats good news that I can just add the volume pots. =)

If the amp is a stereo amp, then a balance control is a really good idea. If they are monoblocks and each has its own volume, you have to manually adjust the volume for each channel- that gets old fast!

@georgehifi Please excuse my limited knowledge, I don’t fully understand the phase angle and stuff.  But to clarify, my X5 has one 12” Eminence driver and one 12” powered Eminence subwoofer by Hypex Fusion NCore plate amp.  

The cross over frequency is 90Hz. AMT - 1 kHz and up
12” mid range - 90Hz to 1 kHz
12” powered subwoofer - 90Hz and below
But that 90Hz to 1 kHz is exactly where the impedance dips to 7 ohm.

actually, I can do a test on my current amp which supports 4 ohm.  And I have been connecting my X5 to 4 ohm.  I did a brief test earlier and I remember there is no difference on my X5.  But I remember there is an improvement to use the 4 ohm tap on my Zu Omen Def.  So, it seems this really depends on the speaker.  Maybe the phase angle is less relevant in a open baffle speaker?  
But I may change speaker or I may sell this.  Definitely having a 4 ohm tap is more future proof, but I am not sure if I want to spend 20% more for the unknown future.  If I happened to buy a new speaker, maybe I should just buy another better matching amp .... decisions,  decisions ....