Here is my less than informed take; Mica and others can speak much more authoritatively on this subject than I, and of course Michael has owned lots of older Series basses and has immense first hand knowledge, but perhaps this will help some and not be too far off base. I have yet to own a Series instrument, but have followed this issue closely. As a generalization, older Series basses will have some degree of hum and noise. One of the reasons for this is the significant and ever increasing amount of electronic transmissions in the air around our basses; so that a bass that would have been quiet in 1974 might be noisy in 2004. From what I understand, the amount of hum or noise can vary greatly depending on where you are playing, some players may only experience a problem in one particular club. The amount of hum or noise can apparently vary widely amoung basses, even of the same year. I do not think there is a particular build date which says whether you definitely need an upgrade or not. Also, it is my understanding that Series electronics are individually hand calibrated, so I doubt that there would be a do-it-yourself upgrade; apparently the adjustments needed for any particular bass will be unique to that instrument. When considering an older Series bass I always ask the seller about hum and noise and whether the bass has had an electronics upgrade. It is also worth mentioning that on the back of Series basses there are trim pots for the purpose of adjusting the hum cancellation; it could be that in some cases, the seller may not have the trim pots adjusted optimally. I could be wrong about all of this, I'm just guessing; but that's my take from trying to make sense of this subject for a while.
