Okay, I just finished opening it up and taking a look. It didn't go as smooth as I had hoped, and I ended up cursing your name the whole way, Jeroen, for putting the bug of curiosity in my mind because I encountered a series of mishaps that added another hour to my time.
First I plugged her in again to double check the layout/configuration. The active treble pot was now crackling like crazy and needed tightening. No problem, I thought. So I unscrewed the teeny tiny knob screw holding the knob on the pot shaft and it proceeded to bounce away and under the shop table somewhere. I'll get it later.
I then tried to pull off the knob and it wouldn't budge. I tried everything - a wrench, flat head screwdriver, etc. - nothing. I started thinking the idiot who sold me the bass had crazy glued the knob to the shaft. Fifteen minutes later it turns out I'm the idiot because the knobs have 2 different tightening screws and I didn't see the second one
(When was the last time you saw two tightening screws on a knob?).
So I got it off, tightened the pot nut, and plugged her back in. More cracking! Turns out I had a bad cable. I replaced the cable - now I dead silence in active mode! Turns out the battery died at that specific moment.
Replaced the battery. FINALLY everything's in order. Ugh! Took me a half hour to find the lost screw under the table, but it's back on the bass now.
Anyway, back to the original discussion. I looked under the hood. What a rat's nest! I found the PUP blend knob actually has 2 pots stacked under it (not a true stacked knob/pot, just another pot sitting on top of the other one), with the top pot listed at 500k. I'm not sure if this is original or not, but the pot under the 500k top pot is definitely original and still wired up, so I have to assume that each PUP's got it's own pot. (The top one might be a replacement, however). I also checked the output of passive to active again, this time through my amp. (I previously did it through headphones). With all knobs turned up fully and the 3 band active knobs flat (mid point), the output is about the same, though I'd say the active mode is just a tad thinner sounding. Adjusting the EQ in active mode to cut frequencies is fairly noticeable through all stages, but when boosting EQ bands the sweep is more subtle and less apparent until you get to the final 1/4 -1/8 turn where it definitely boosts the levels substantially. I'm not 100% sure on this, but I believe these were designed to do this with that added signal boost in the final stages.
Mine has the Super P5 and J5 PUPs, with a humbucking P style PUP in the neck and a single coil J style in the bridge. The 60 cycle hum on the J PUP soloed is fairly noticeable and even when PUPs are blended it's still there, so there's no hum canceling as on a standard J bass when PUPs are blended. Yours has the Super 4 single coil PUPs in both the bridge and neck positions, which are single coils, correct?. Does the hum cancel with both PUPs on?
One other pet peeve of mine: also note the righty truss rod covers on these basses (as well as the Roadsters). It never ceases to baffle me how a company can go the distance to tool together a lefty instrument, yet leave a relatively simple detail out like left handed truss rod cover! is it that difficult to do when you've tooled for everything else lefty?