Hi Lars,
I hope you found a solution to your problem. I've used two of the above-mentioned methods, and they both work great(must admit I still haven't tried the "Ditch the Fender"-suggestion. Sorry, guys!!!)!
1. A Lehle pedal(or another brand A/B-box with adjustable gain). Lehle makes several great pedals. In addition to the one Florian suggested, I would also recommend the Parallel-L. It has to channels, and a volume on Channel B, so you can plug your passive bass in there, and match it to the level of your active bass. Another great thing about that pedal, is that it has a Mix/balance knob. If you like using overdrive/distortion on your bass, you can put the overdrive through channel B and then mix the clean and the overdrive signal so you can keep more of the bottom in your signal while getting as much overdrive as your pedal will deliver. It's also great for blending to pick up systems on an upright.
2. A clean boost pedal. Several brands make these.
Good luck!
