If the nature of your problem is just the output levels of your different basses, then the simplest solution would be to adjust the output levels of the instruments to all be the same. Because it seems that some are passive and some active, this will have to be done in a passive manner to suit the passive basses.
Find the "quietest" bass on maximum volume, and adjust the volume controls on the others to match this volume level. Now measure the volume pot with a multimeter between top to centre and between centre to ground. Then get a new volume control of the value of the measurement between centre and ground and a resistor of the value of the measurement between top and centre. Replace the volume control with the new one, and wire the resistor between the wire that came from the top of the old volume control and the top of the new volume control.
What this mod will do is effectively restrict the maximum volume on the louder basses to the level of the quietest. Remember, the vaules of the new parts you buy don't have to exactly match the measurements, you just have to be close.
Hope this makes some sense. There are other things you could try such as adjusting pickup heights etc which will also change the output levels, but may change the sound too.