I'm in a similar situation, new JSV3, weak pup's and very hissy preamp. For the price (I got a deal at $199), I like it enough to keep and eventually upgrade the electronics after I play a few more gigs to get the funds. In the meantime, I'm experimenting with passive mode to at least get rid of the hiss. The non-eq pots on this one are volume/blend, not volume/volume. So far I've removed the battery and jumpered the volume output to the output jack, without changing/removing the existing preamp connections. What I find now is the blend knob goes from max output at one end, to near zero in the middle, to max at the other end, so it is effectively a selector switch to have one pup or the other, and not useful for blending at all. Is there some other wiring change I can make with the existing components to restore a useful blend function? Would disconnecting the (unpowered) preamp make a difference?
Question for the OP (or anyone else who has done this), did you change to the EMG DC's and BQS, and how does it compare to the stock parts?