Hmmmm... couple of questions, some which may seem silly but want to rule out the easy culprits;
- Is the input pad disengaged?
- What, if at all, is the compressor set at? You'll want to turn this all the way off, since the compressor is actually ahead of the input gain stage.
- How is the pickup height on your J-Bass set? This can make a HUGE difference on any bass, but most especially so on older and passive instruments
Too much gear, too little skillz.....