Place a microphone stand directly in front of the amplifier grille. For a punchy and powerful sound, position the microphone holder, or "collar" so that the microphone diaphragm points directly toward the center of the speaker. For less microphone sensitivity, position the microphone at a slight angle to the speaker. Typical bass amplifiers have a single speakers, but on the rare occasion that you have to record a stereo bass amplifier, position the collar in between the two speakers.
Position the second microphone approximately ten feet away. This captures the sound of room as well as the bass amp, but because of the proximity relative to the other microphone, the sound is a lot less direct. Use ten feet as a starting point for finding a suitable position, but the distance is not absolute. The correct distance is the one that creates the best sound. Factors such as room size, microphone type and amplifier volume are all influences.
Slot a bass microphone into each collar. Some bass microphones, such as the AKG D11 has an integrated stand, so there is no need to set up a microphone stand.
Connect the female end of an XLR cable to the bottom of the first microphone. Connect the other end of the cable into the "Channel 1" XLR socket on the mixer. Connect the other microphone into "Channel 2."
Plug a pair of headphones into the output of the mixer. This enables you to monitor the sound of the microphones without that sound feeding back into the same microphones. Gradually increase the gain so that it is as high as possible without causing the meter to flash red. If it does flash red, reduce the gain slightly to prevent distortion on recording. Once you've set the gain level, gradually increase the "Master" volume dial so you can hear the sound of the bass. Tweak the equalizer dials, typically labelled "Low, "Middle" and "High" to shape the tone of the bass.
Press "Mute" on channel 1 and gradually increase the gain of channel 2. Tweak the equalization dials again. Pay attention to the ambience of the sound, rather than the body of the sound. Move the second microphone around the room until you find a position that gives you a balanced, ambient sound. The close microphone will deliver the punch and weight that you'd expect from a bass instrument. The room microphone therefore should be configured to pickup as much resonance and ambience as possible without causing phase interference with the first microphone.
Press "Mute" on channel 1 again to unmute it. Tweak the faders of each channel respectively to set a volume balance between the two. Once you're happy with how the two microphones sound on the mixer, you're ready to make a recording.