Yamaha makes really high quality stuff for the price. The pickups in my TRBX304 were great, loud and punchy; the ones in my 604 are absolutely fantastic and I wouldn’t think of changing them out.
The BB734s I have tried sounded fantastic to me, I wouldn’t change a thing.
Several of my favorite bassists use them stock, both live and in the studio, and they sound great. The YGD pickups are just really good.
As @howard stated, there is no reason to replace pickups if you’re happy with the sound.
In my case, I was not replacing pickups on a high quality bass, but rather on a very inexpensive bass (Squier Affinity Series PJ Bass) in an effort to make it sound more like a high quality bass. I also replaced the bridge with a high mass bridge.
I have a set of Hepcat 62 for my Japan PJ. Seymour are great too. My luthier has lined the cavity with copper foil. This is something any bassist can do on his own because of the many tutoz on youtube. Also replacing the cabling by quality wires could help. I am unable to help about the problem with the darkglass vintage. With the cavity once copper foil lined, maybe the hum will decay to an inaudible level. Signals radiating even at a very low level can add up hence interfere with the ground. Please investigate and let me know. Cheers Riastarthae