Bass signal competing with phantom powered signals?

I have played two different basses equipped with bartolini soapbar pickups and active/passive onboard preamp. In certain situations with the bass connected to a PA system it seems that the signal from the bass "fights" to stay alive--If I play hard and maintain a very strong signal, it gets through. If I play lightly, the signal can get choked out altogether, as if I were playing through a noise gate. In one situation I was touring with a 4 piece band with a mandolin player with everybody going through a PA. When the mando player had his instrument plugged into his outboard preamp and into the PA, the signal loss problem occoured, but not always. In another situaiton I plugged the bass into a mixing console with phantom power and experienced the same problem, but when I plugged into a grace 201 preamp first then into the mixer, problem solved. The signal cut out happened whether or not the onboard preamp was on. What gives??? Help me out techies.