[font=trebuchet ms]Feedback happens when soundwaves from the speakers cause vibration in the instrument, which adds to the signal going into the amp, coming out of the speakers, causing vibration in the instrument....

Short of trading your bass in for a solid-body (less acoustic sensitivity to sound around it), you have to find a way to isolate your instrument from the speakers. Pointing the speakers a different direction, moving further away from the speakers - whatever it takes.[/font]