The output taps on Mac amps (like the MC352) with output transformers adjust the output stage for the best impedence match with the speakers. It shouldn't hurt anything to try your speakers on each of the taps, and see which sounds best.
BTW, I just got an MC352 from Audio Classics last week, and love it. I'm using it to drive Martin Logan Ascents.
BTW, I just got an MC352 from Audio Classics last week, and love it. I'm using it to drive Martin Logan Ascents.