Hey Steve,
Not every instrument needs the "upgrade". Our big single-coil pickups "hear" a wide audio spectrum, which is why the sound is so massive. But because the hum-cancelling coil is physically a couple inches from each pickup, sometimes there is some RF or hum that gets through the cancelling network. For many years we all learned to live with the understanding that in some locations we might need to stand in a certain direction to keep the noise floor down.
Then our resident Wizard Ron Wickersham came up with a way to fine tune each individual preamp board (in each individual instrument) reducing the noise floor considerably. But as I say, not every bass needs the tweak. If noise becomes an issue for you then consider getting on the wait list for that mod.
Regarding the DS-5's mono output... The furthest counter-clockwise position of the pickup selector should be "mute". Next position is Bridge pickup only, then Both, then Neck pickup only. The "Both" position should be an additive sound, simply a sum of the outputs of the two pickups. If your "Both" position has an odd hollow sound to it, it's possible one of your pickups has been plugged into the preamp board out of phase.
If you get the chance, pull off the brass back panel and post a pic of your preamp board. You can do that the same way as above - with a link - or reduce the image to under 1MB and attach it to a post in here. Click 'Preview" to get to that attachment page. And you won't see a preview of the pic until you actually post the message.
Enjoy that thing!
Jimmy J
