My speakers are rated at 8 ohms but the spec indicates the "minimum impedance" to be 3.1 ohms. I tried the 8 ohm taps first and like you, found the speakers really came alive when I moved the wires to the 4 ohm taps. My dealer thought that was fine and the amp runs "cool as a cucumber", no problems over the 7+ years I've owned them!
McIntosh Output Autoformer, Sabrina X and Serge Gainsbourg
I’ve put a good deal of attention into room acoustics, components, sitting position and so on. My assumption was that my Wilson/McIntosh dealer had setup my MA12000/Sabrina X properly. That said, I did have the common sense to adjust my listening position and the above changes. What I never checked was which of the Autoformer output terminals were being used. The Sabrina X gets down to around 2.6 ohms at 135Hz, so I was a bit surprised to see that they used the 8 ohm terminals. Moved over to the 4 ohm terminals and put on “Histoire De Melody Nelson”, which is a great voice in the room recording. The live, in the room presence was amazing. I can’t believe that I’ve been listening with such a handicap. So much for the expert in home speaker setup. Give the above album a listen. Really nice recording released in 1971.