It's possible to use a bass guitar equipped with a 13 pin pickup system (e.g. GK-3B) with the GR-55. When doing so, you'll want to switch to "Bass Mode" in the GR-55. Here's how:
1. Press EDIT.
2. Press the PAGE right button to choose the SYSTEM tab.
3. Press the cursor right button to choose the BACKUP/INIT icon, and then press ENTER.
4. Press cursor right to choose GUITAR <->BASS, and then press ENTER.
5. Cursor right to highlight "OK," and the press ENTER.
6. Restart the GR-55.