I'm not an EBMM guru either, just talking about what I have seen.
The MM pickup, in series, is definitely found on active SUBs and sounds awesome. I have not seen a passive one, I guess they were not so popular in this part of the world, but every 4string SUB I have been able to check (all three of them!

) were wired in series, so i assumed that was the norm.
Anybody here has a SUB wired in parallel? My SUB5 was, confusingly... go figure.
I meant to rewire my SUB 4 to parallel, for maximum Stingrayness... but it sounded so punchy that I kept it as it was.
People have installed series/parallel and preamp bypass switches together for a long time and it doesn't seem that any combination is "bad". The electronics info may make sense... but when it comes to guitars, it doesn't have to make sense. I mean, who would have thought in the early 50s that distortion would be a desirable feature in an amp?
My new SUB will be with me early next week... I will check that one and report back. If it makes any difference, the other SUBs were all early models, all 2003 in fact. This one is a late one with the compensated nut. We will see.