I recently purchased a U-Retro Deluxe with passive tone control. BestBassGear.com told me that the passive tone control would not really work at all with active pickups and even John East himself told me that his passive tone control was designed for passive pickups. I have active Seymour Duncan Basslines soapbars.
Well, I was disappointed but hooked the preamp up anyways. To my surprise, the passive tone control works beautifully in both active and passive mode! In this instance, it didn't matter that my pickups are active.
So there you have it. In case you are wondering, John East's passive tone controls CAN work with active pickups and not just passive pickups. (at least Seymour Duncan Basslines pickups)