First, you need to make sure cancellation is off. Off, boost, enhance, or cut. Cut is the factory default, this will cut off the bass or guitar sound into the unit.

Here's how: press the CD Effect button and hold it down for a few seconds, it should say Bcut or Gcut. Press the up arrow and select between Off (normal sound), BST (Boost), ENH (Enhance), or Cut (remove bass input). This changes the guitar / bass instrument.

Also, another thing to check is to press Setup, make sure Mix is set to zero, default is slid to the cut side.