The "owners manual" for the Bass 400 is simply 4 typed pages - shoot me an email and I'll send a copy over. firstname.lastname@example.org
Occasionally the switch gets "stuck" halfway and can be the cause of signal loss, but more likely its a component related issue.
When the EQ is engaged, is there any hum? Does the hum change when the sliders are moved up and down? Or is there no signal at all? This is the more likely scenario, and will need to be seen by a technician.
Let me know if you have any questions or need assistance in any way. Thanks!