Yes. The problem is, from an amp's perspective, low impedances are hard to drive - not the high ones. Many (conventional dynamic box) speakers have a low impendance in the bass and a high impedance in the upper mids and highs. The average is a very rough 8 ohms so that is what is published in the manuals and brochures.
However, all the amp really sees is that 4 ohm bass impedance! So technical people will say, use the tap that corresponds to the lowest impedance of the speaker. In 90% of cases today this is about 4 ohms - especially true for B&Ws.
In the old days, speakers had a much higher impedance. There are several complicated reasons for this but to keep it short, they would often average 8 or even higher. If you look at old tube amplifiers, they all have 16 ohm taps and some even have 32 ohm taps! With the advent of cheap high-power solid state amplifiers in the 1970s, speaker designers gained some freedom and have since settled on a much lower typical impedance curve (again, for many reasons). This is why modern tube amplifiers only have 4 and 8 ohm taps - 16 ohm speakers are basically extinct and REAL 8 ohm speakers are not far behind.
Arthur
