In the words of MXR:

The MXR Bass Preamp combines a pristine bass preamp with a
studio-quality Direct Out in a Phase 90-sized box, providing clear tone
without hogging precious pedalboard space. Use it to add a new voice to
an old passive bass, sweeten up any amp, or put some special sauce on a
particular song. Dial it in just the way you want it and send it
straight to the housepre or post-EQwith the built-in Direct Out.