Update:
I wanted to update this thread so y'all know how it turned out and in case anyone else decides to attempt something similar with a Yamaha TRBX1004.
The electronics were updated to a Nordstrand 2b preamp. It came pre-wired with a stacked bass/treble, but I broke them onto their own pots so that all 5 holes in the front of the bass were filled (volume (p/p active/passive), blend, bass, treble, tone). I was originally attmpting to install a Glockenklang preamp but the pots that were installed on it were the wrong size for the holes. The cavity on this bass is routed out, then there is a recess for each of the pots to sit in. The pots on the Glockenklang were too broad to sit in the recesses and when it was sitting on top of the recess, the shafts were too short to attach to the front of the instrument. The pots on the Nordstrand worked better for this, but the volume pot gave me some problems and I still don't think the shaft is straight. If I were doing this again I would order different pots with longer shafts (Gibson-style?).
You can reuse the battery compartment but the hard plastic-type of clips barely fit. You will likely need to reuse the output jack as well; none of the others I tried were going to work.
The pickups fit better than expected. I bought some of the pickup mounting foam from BBG and cut strips to the
width of the pickup routs, then attached them to the back of the pickup. I also stuck a piece on each end of the pickups (you can see these in the pictures). All the foam helped center the pickups in the routs. Unfortunately the bridge pickup still didn't mount quite straight, but it's good enough for my purposes (I'll be surprised if this instrument ever sees a stage again). I did need longer pickups screws (1.5") for these BC-size pickups versus the screws that were installed. The pickups wound up looking better than I expected them too and, with all the foam I used to install them, are surprisingly solid in place.
Was it worth it? Maybe. The original pickups weren't bad, but the original preamp was terrible. I learned a lot along the way. One of the major advantages of buying American (or probably European as well) made instruments is that things tend to conform to certain standards that we all know and understand. So the problem with updating one of these Korean made instruments is that pieces don't necessarily conform to standard sizes (like pickups in J, EMG 35, or BC sizes, or the control cavity recesses), which is too bad because the rest of the instrument is fine! It plays nice, stays in tune, and sounds pretty good! But trying to push it to the next level took some patience and creativity.
I'll be doing similar operations to two more basses in the coming days: a Schecter Omen-8 which I assume I'll have similar issues with (installing BC-sized Nordstrand Big Blades into different, non-standards pickup routs, plus updating the preamp (the plan is a Microtubes Tone Capsule, but we'll see if it works (if not I'll use the Glockenklang that didn't get used in the Yamaha (sorry for the embedded parenthesis (not too sorry, I dig avant-garde sentence structure (and I'm not working right now so I don't get to unleash this kind of madness on coworkers)))))), and a G.Gould that currently has EMG 40s and an EMG preamp. I assume the G.Gould will be much easier to modify than the Schecter, but I'm glad I got the practice on the Yamaha.
