There hardly are any real 8 ohm speakers in the world - and especially not any B&Ws. If you take a look at speaker impedance plots, the magnitude is often all over the map. The only speakers I have seen with a rather flat impedance are transmission line types (like some of the VS) and Coincident among other little companies that make single driver speakers and special crossover designs.
So, the reason McIntosh said use 4 ohm taps is because your speakers are around 4 ohms (and below I believe) around the port tuning frequency. So they are not really 8 ohm speakers...B&W is just duping you like many other speaker manufacturers with their "nominal" specs.
So, the reason McIntosh said use 4 ohm taps is because your speakers are around 4 ohms (and below I believe) around the port tuning frequency. So they are not really 8 ohm speakers...B&W is just duping you like many other speaker manufacturers with their "nominal" specs.