I have exhausted myself trying to figure this out, and have brought my bass in to a local repair shop numerous times. I built a bass using a Music Man SMB 4A passive pickup and a SJB-3 pickup for the bridge (using 500k pots, 2V, 2T and 3 way switch). Both pickups sound great but they are not very loud compared to my P-bass. Using a multi-meter the SMB 4A gives me a reading of 7.5K and the SJB-3 gives a 14K reading. Together they come in around 10-11K. My P-bass has an output reading at 10.5K. The volume, however, is considerably different. The output is comparable when my built bass is at a level 7 on my amp and the P-bass at a level 2. There are times where the volume drops even more spontaneously and the tone gets kind of muddy. I should mention that the way it was wired, when both pickups are on, if you turn down one volume knob both pickup volumes decrease instead of independently. (It is wired in accordance with this diagram: Bass Instructions - Google Drive, with the music man pickup wired in series). Can this issue be fixed? Is there a way to test if volume is being lost through the pots or switch?