@fsonicsmith, I think you are addressing me, not George since the quote is mine.
The DeVore is not a four ohm speaker; as far as their website is concerned, they don't make any. But it works best on your four-ohm taps. That is quite a bit different from the speaker actually being four ohms. It simply means that combination works best for you- and as you noticed, the speaker cable is a bit critical in the overall scheme of things.
Can you unequivocally state that using the 4 ohm taps on any tube amp,
including my ARC Ref150se, ipso facto results in brightness and
hardness?
Not- at all, and if you got the impression I said that, I didn't. What I did say is that the use of four ohm speakers results in higher distortion from any amp- and that the distortion is such that is perceived as brightness and hardness. There is a distinction here- you are not using a 4 ohm speaker.
Using the four ohm tap gives you the lowest output impedance available from the amp and for some reason that works. FWIW, usually you loose some bandwidth using the 4 ohm tap; IME its usually in the bottom octave.