I have two SM81 mics and I was wondering exactly what each setting of the switch on the microphone means as well as the best application of each of these settings.
The attenuator ring is located just below the mic element. It inserts a 10dB pad when rotated. Use this only if you are recording extremely loud sounds, like a guitar amp at maximum volume with the mic 2 inches away. 99% of the time do not use the attenuator switch.
The three position roll-off switch should be set for flat when recording very low frequencies.
The middle position (slight roll-off) should be used for most recording situations. This helps reduce rumble from unwanted low frequencies.
The steep roll-off position will indeed affect the low frequency response. This might be a good position to use when recording cymbals as there are no desired low frequencies that radiate from cymbals.
As a test of the roll-off switch, have a male friend talk into the SM81 while you listen to it via headphones. Note how the voice changes with each switch position.