I started to pan the bass in the middle, but then changed to pan it a little bit to left to give it more separation. In the lowest frequency ranges the position of the sound source can't be heard, but in bass guitar is also frequency ranges, which can be positioned in the sound picture. So my basic sound picture tries to imitate a real acoustic band on the stage: singer and solo players, but also the drums (because they're spread already) in the middle. Then low freq range instruments (bass, organ, cello, rhythm accordion, piano etc.) ca 25-30 % to left and right. Then the rhythm keepers (rhythm guitars, banjos etc.) ca 50 % to left and right. Then finally the backing melody instruments (mandolin, banjo, BiaB melodytracks) further on the both sides. I very often also duplicate the vocal track and pan it 15 % left and right. Finally I use also separation in the depth of the sound picture: I put delay to the backing band in reverberation about 7 ms, while the vocalist and solo players only ca 2 ms. So it's like in old fashion concert, where the band is on the back stage and the solists infront are heard first. All this placing to separate the sound sources as much as possible both in horizontal and also in depth. The nature law says, that in the same freq you hear only the loudest sound and it's masking the others. Same goes with sound pulse: the first one is masking the later ones. It's interesting to hear your comments on that placement in the sound picture.