The gain trimmer on the NTMB or NTBT preamps can be replaced with a potentiometer so that you can adjust the gain from the front of your instruments.  Simply remove the existing trimmer by removing the heat shrink tubing and unsolder the wires and resistor from the trimmer. Use a 10K linear pot for the gain adjustment and a 2.7K resistor for the reference.  Alternately you can use a 5K linear pot with a 1.5K resistor.  Or you can use a 50K linear pot with a 15K resistor. See the attached file for the hookup details.  Cover the resistor with heat shrink tubing to ensure that it doesn't short out on anything else in the control cavity.