We had available two B6 microphones and I assume (that word again) they are the standard 6 mV/Pa units since the overload point that we found was close to the specified values. We wired the two mics with a 10k resistor from tip to sleeve of the MM400 connector, which is the wiring recommended by Countryman. We found that the B6's overloaded at between 114 and 117 dB spl and that value is essentially the same as their spec sheet value of 118 dB spl. Our sound pressure arrangement was more practical than exact. Since the microphones swing their entire bias supply of 500 uA at the 114 to 118 dB spl input, they are fairly hot mics. This 10k wiring will develop 1 Volt pk at the normal input of a MM400a. The maximum level that the MM400a will handle before limiting is 0.42 Volts peak. I want to emphasize that this is limiting and not clipping. The signal will still be clean just compressed. So a B6 at maximum signal swing can drive the MM400a into 7 dB of compression. However the B6 itself will start to clip before the transmitter audio circuits begin to clip.
To reduce the gain of the standard B6 4 dB, put a 4k resistor in place of the 10k across the B6 leads in the connector, i.e., from the center pin of the MM400 connector to ground. Three dB is due to a reduced load and a dB or so due to reduced current. This will get the max output of the B6 close to the max non limiting input of the MM400a. If this is still too hot and you are uncomfortable with running the MM400a at minimum gain in loud situations, then I recommend the reduced gain B6 which is down 10 dB from the standard unit.