Old thread, but I know folks like myself look up older threads, so here is my experience.
I picked up an older SR-905, through neck, through body strings. I actually love the MK1s in the SR500s, but this has a maple body and it was way too bright. I spent about 6 months researching and decided to do the Pup upgrade first. I actually had a Nordstrand DC5 lying around I tried in the neck position and it was incredible sounding, but it was 19 mm spacing so the allignment was off. I decided to go with P2 sizing simply because it opened up more possibilities. I had decided it was a tossup between Reed James Neos and Nordstrand Dual Blade Neos, when lo and behold Bestbasssgear has a huge sale, and had a pair of Bartolini Quad Coil P2s at less than half price, so I took the chance (normally $166 each).
Bestbassgear screwed up the delivery (which they did with 2 of the last 3 orders) and delivered 1 quad and 1 dual. Got them to overnight the other quad, but in the meanwhile I installed 1 quad at the neck position. I won't dwell on this, but the only reason I will buy from them now is when the price is right.
I installed in series, and the pickup totally overpowered the Mk1 at the bridge, so my guess is the MK1s are in parallel internally. Rewired in parallel, so I had one Quad and one MK1 and could roll fully to either. I also did the reverse to see if the impact on the sound was the same. Folks have previously talked about the sound difference being an evolution rather than a revolution, and I can tell you that was not my experience with putting in the quad. When I rolled to the quad the the sound was immensely better, and the remaining Mk1 (in both configurations) now sounded hollow and tinny. I put both MK1s back in temporarily to make sure it was not my imagination.
In short the Bart quads sound incredible (expensive normally but worth it) and the bass is transformed with no new pre-amp, this is now my main fretted bass. Wiring in parallel for each pickup gives it a highly defined sound. The next step with be a switch to let me change the Barts so I can isolate each coil (which I can do noiselessly since it is quad coil).
Pete Greaves