One of the amplifiers used in the Stereophile review of the Sabrina was a Mac 275. The reviewer first used the 4 ohm tap and thought it sounded fine. Wilson's Peter McGrath mentioned that Audio Research amps sounded best from the 8 ohm tap. The Mac was switched to the 8 ohm tap and both McGrath and the reviewer preferred the 8 ohm tap. Therefore, setting the output tap to correspond to the lowest impedance dip may not result in the best sound. Even though the reviewer acknowledged the solid state amps provided deeper bass, he preferred the overall sound best with the tubed Mac 275.
Comparing the anechoic frequency response whereby there is a pronounced upper bass hump to the impedance magnitude indicates this is where impedance is lowest. A tube amp will likely produce less drive in this range which may minimize the bass hump. It was also noted the Sabrina's bass Q was on the under damped side which a tube amp will tend to make more pronounced which may even things out.
I prefer a quality tube amp for its more natural sound. Have fun in the auditioning process.