On some ARC amps, the feedback is taken from the 4ohm tap. I find it sounds the best. When a speaker manufacturer says their speakers' impedance is 8 ohms understand that this is not a constant across the frequency response of the speaker. An 8 ohm speaker can dip to 4 or 5 ohms at it's minimum. This usually occurs after it's resonance peak and is a function of the crossover and how each driver is compensated. It's easy enough to try both taps and see which one you prefer. This should not cause any damage to speaker or amplifier.
