Here's the things I've learned: if there is a wall nearby or a car try to get the jam to be there and stand where the sound coming off the back of your bass reflects to your ear. The more relaxed you are the louder you can be and better tone. Learn to really pop your strings with the side of your finger. Generally speaking notes on the bigger strings half way up the neck like the "a" note at the 7th fret on the d string will be louder than the a note on the g string so when the banjo is soloing go there. And last but not least find and buy an old and loud bass.
Happy picking!
Happy picking!