Here's what Wadia says:
All switches open = 0 dB / 4.25 Vrms
Switch one closed = -6dB / 2.13 Vrms
Switch two closed = -12dB / 1 Vrms
Switch three closed = -18 dB / 0.5 Vrms
Switch four closed = -24 dB / .25 Vrms
The factory setting is 2.13 Vrms
All switches open = 0 dB / 4.25 Vrms
Switch one closed = -6dB / 2.13 Vrms
Switch two closed = -12dB / 1 Vrms
Switch three closed = -18 dB / 0.5 Vrms
Switch four closed = -24 dB / .25 Vrms
The factory setting is 2.13 Vrms