My understanding is that the S8s are ruler flat when driven by a solid state amp.
I doubt that, Bifwynne. With a SS amp, with the output impedance being very low (well into the sub-1 Ohm region), the amplifier is much less perturbed by the roller-coaster impedance & phase response of the speaker. The low output impedance of the amp allows it to sufficient source/sink current into the speaker to give you good sonics & dynamics (assumption is that the power amplifier has a robust power supply that is able to more or less double in wattage for each halving of the speaker impedance). So, the speaker will still have its roller-coaster impedance & phase responses but you will hear less of it with a SS amp.
Perhaps, using the 4 ohm taps lowers the amp's output impedance (maybe .5+ ohms) enough to smooth out the speaker's actual FR to approach a near zero output impedance of a SS amp. Dunno.
No, that's not what happens. The output impedance of a tube amp is dictated by the # of the tubes used in parallel in the output stage & the amount of global negative feedback used by the designer (each & every electronic output device - tube or transistor - inherently has local negative feedback in it. You cannot get away from this local feedback. To quote Nelson Pass in one of his White Papers "show me an electronic device & I will show you the local negative feedback").
Tubes usually have a very high output impedance relative to a speaker hence you have to jump thru some hoops to make a tube amp drive a speaker directly i.e. without output transformers. People like Atmasphere & Berning do that. And, back in the old days there was Harvey Rosenthal (?) who came up with the 1st zOTL amp (I think I'm remembering this correctly??)
The output transformer is put in place between the tube power output stage & the speaker input to buffer the tube amp from the wild swings in the impedance & phase of the speaker.
On the primary side of the output transformer, the tube power output stage sees a constant impedance. By working into a constant impedance, there is optimum power transfer from the tube output stage into the output transformer primary windings load impedance. So, the waters (if you may) are calm/serene.
On the secondary windings side of the power output transformer, the waters are rather choppy due to the speaker impedance & phase variations vs. freq. Several output taps are provided to match the speaker impedance such that there is more optimum power transfer between the secondary windings & the speaker input.
Some speakers are categorized as 8 Ohms speakers by their manuf. Then, use the 8 Ohm tap. Other are categorized at 4 Ohms speakers. Then, use the 4 Ohm tap. Still others are categorized as 6 Ohms speakers by their manuf. In this case, one needs to try both 8 & 4 Ohm taps to see which one is better. For example, the RM10 likes to use the 4 Ohm tap for a 6 Ohm speaker. The way this amp is designed it generates more power into the speaker. (I think I'm remembering this correctly??).
So, changing the output transformer tap merely provides a way to "tune" the power transfer towards more optimum between the tube power output stage & the speaker input. The amplifier output impedance does not change in the way your sentence reads.
FWIW.
The S8s are the darlings of the reviewer community, especially because of the Be tweeters and top grade engineering, R&D and QC that goes into their manufacture.
so what?? are these reviewers your friends i.e. do you trust their judgements & opinions? Do you know them personally? have you been to their house & Listened to their resp. systems? Are their music tastes aligned with yours?
If yes, to all the above questions, then, I would buy what they recommend. Otherwise, it's just a review to be taken with a grain of salt.
Who cares about the Be tweeter, the fit & finish & the QC when the speaker manuf does not possess the fundamentals of speaker design??? Get the physics right then let's talk Be tweeter, the fit & finish & the QC. Otherwise, this is all marketing hype disguised to hide fundamental flaws in the speaker performance as you have found out the hard way. Atleast, this is my take on the matter. YMMV.