I had a similar tone goal for my TRB5, although I felt that it sounded more modern than I wished, rather than more "dull". I wanted to back off the zing just a little.
When I went through this with my TRB5, I learned that (at least on the TRBs) Yamaha has really done their own thing with the electronics on their basses. The preamp was totally proprietary and pretty much could only be used with the OEM pickups, so I had to replace both. Also, I could identify no aftermarket pickups that fit the OEM routs, so I had to cut some little blocks of ebony to occupy the open space left by the Bartolini pickups I used.
Oddly, despite the fact that the Barts I used are supposedly darker than average, and the Aguilar OBP-3 I used has a much lower output level than OEM, my bass still sounds very much like a Yamaha. I expected more of a sea change in tone than I got. If I had been able to hear the "after" tone, I most likely would have avoided the expenditure.