Preamp and impedance question

You can put a volume pot on the amplifier, the only reason most amps do not have one is it limits their market to customers with only one source. What you have to take into consideration is what input impedance your source wants to see. Anything above 10k ohms should work fine. I would go with a shunt volume control as it only puts one (series) resistor in the signal path and I would use a series resistor of 15K ohms, high enough for a respectable input impedance through the volume steps and low enough to keep out noise. You could also include a balance control if you want, just place it upstream of the volume pot.

If your amp has a 4-ohm tap, you can use that with no problem. The power output does not change, but the current draw will be higher as the output xfmr tries to maintain voltage at the lower reflected impedance (power is consumed at the speaker, NOT the amplifier). Any limitations on the amplifier will show up here in the form of voltage drop and distortion as the tubes are asked to work harder. However, it should not affect anything if you listen at reasonable volumes.

@gs5556 
So, my understanding is that, as long as I listen in low to medium volume, paying extra to get a 4 ohm tap will not have audible improvement over the 8 ohm tap, am I correct?

paying extra to get a 4 ohm tap will not have audible improvement over the 8 ohm tap, am I correct?

Yes it will the bass will be tighter/controlled with the 4ohm than the 8ohm tap, if the speakers are 4-5ohms
Go the $500 extra you can't go backwards once it’s done, and the amp will have far better resale value down the road.
Cheers George

The output impedance of tube amplifier with a transformer is a function of the (output impedance of the amplifier section multiplied by the turns ratio) ^2  (squared).  The 4 ohm tap has less total turns, so it has lower output impedance (higher damping factor).  On the other side, changing the tap, changing the turns ratio, reflects a different load back to the amplifier, and coupled with the transformer, that can have some impacts on the flatness of the frequency response at the ends of the audio spectrum and the phase. All depends on the overall design.

In terms of selling the amplifier, "clones", especially something that is a production clone, are of limited resale value, so keep that in mind when looking at what you spend. The transformer is a huge part of the implementation. You are building something "like" a Luxman mq-300, but certainly not a Luxman mq-300.

You can ask Spatial audio if they can supply an impedance graph/chart. There really is no excuse for not being able to supply one. From there, you could make a decision at least on this speaker. They claim 8 ohm, and there is no reason to doubt that. Rather large mid-bass, so the impedance could dip, but 8 ohms is still likely best for overall performance.  Different speakers, of course, will behave differently.

@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