Charles1dad -- Bombaywalla makes a fair point that many fine speakers have roller coaster impedance and phase angle plots. As I posted above, even the "mighty" Revel Ultima Salon 2's impedance plot is a bit of rock and roll.
OTOH, I think Bombaywalla's comment about me being in "sonic hell" may be a bit extreme. As I also posted above, the ARC Ref 150 and VS-115 amps were bench tested by different labs. Yet the results were consistent. As Ralph and Al explained, the use of NF reduces the output impedance of the amp, thereby making them more amenable to driving speakers with roller coaster impedance and phase angle plots. That is, somewhat SS-like.
Al also made a very important comment that even though the ARC amps FR plots were pretty flat at the speaker input interface, the most important stat is what comes out of the business end of the speaker. In the case of the S8s, they are rated, and have been bench tested to be very flat -- when driven by SS amps.
I surmise that by using the 4 ohm tap on my VS-115, which as Al thinks might reduce the output impedance of the VS-115, the S8's FR output might approach that achieved with a SS amp. Based on my subjective experience, since switching over to the 4 ohm taps, I think the S8s have a tighter bass response and are less forward as compared to using the 8 ohm taps.
BTW, I seem to recall that ARC uses Wilsons to voice their amps.
Bottom line: if I were to switch out my speakers to the Revels, I would probably also switch to a high current/high power SS amp, as recommended by Revel. At least Revel was honest enough to tell the consumer the type of amp that should be used.
P.S. Bombaywalla -- I think it's unfair to imply that the S8s are junk. I have no intention of starting a speaker war, but suffice to say, I think if properly tweaked and set up, they do a very credible job.