In the words of MXR:

The M288 Bass Octave Deluxe is a dual-voice octave pedal that offers
analog tone, true bypass, and a variety of tonal options. MXR’s 18 volt
Constant Headroom Technology provides high headroom and superior
tracking. Use the Dry knob to mix in your direct signal, and hit the
Mid+ switch to add up to +14dB of internally adjustable mid frequencies
(400Hz or 850Hz). Finally, mix in two separately controlled octave
voicesGrowl (for throaty tone) and Girth (for deep tone).