This passage from Ballou's (1991) New Audio Cyclopedia states that the filter is engaged when the circuit is open:

As in many cardioid microphones, the RE20 has a low frequency roll off switch to reduce the proximity effect when close mic'ing. Fig. 1 5-87 shows the wiring diagram of the RE20. By moving the red wire to either the 250 or 50 Ohm tap, the microphone output impedance can be changed. Note the "bass tilt" switch that, when open, reduces the series inductance and, therefore, the low-frequency response.

iLCacciLLo - Is this definitely not correct?