Thanks to Luis and others here for helping me to simplify my setup.
I am now routing a mic (via a mixer) into one input of a DL4. My bass into the other input.
The entire pedalboard (including the DL4) has a bypass switch in front of it (so I can take the whole thing out and get my straight bass tone when I want).
Since adding the mic into the second DL4 input, I notice a major volume drop in my bass (my volume when I'm bypassing the board is much louder than when I'm going through the board and the DL4). This was not the case before I put the mic into the second input - at that point the volumes (bypassed or through the board) were pretty much equal.
Can anyone enlighten me as to what is going on here, and how I might fix it?