You can do a setup up like that with active/passives, but there are a few ways to do it that yeild different results. It could have been wired wrong....
Taken From EMG's Website
"Can I mix EMG's with passive pickups?
It is possible to mix EMG's with passive pickups. There are three possible wiring configurations; one is better than the other two.
Use the high impedance (250K-500K) volume and tone controls. The problem is that the high impedance controls act more like a switch to the EMG's. The passive pickups, however, will work fine. If you have a guitar with two pickups and two volume pots, with a three-way switch, there is another alternative. Use the 25K pots for the EMG, and the 250K pots for the passive pickup. This way you can use one or the other with no adverse affects, but with the switch in the middle position the passive pickup will have reduced gain and response.
Use the low-impedance (25K) volume and tone controls provided with the EMG's. The problem here is that the passive pickups will suffer a reduction in gain and loss of high-frequency response.
This is the best alternative. Install an EMG-PA-2 on the passive pickups. There are two benefits to doing this. With the trimpot on the PA-2, you can adjust the gain of the passive pickups to match the EMG's. The PA-2 acts as an impedance matching device so you can use the low-impedance EMG controls (25K) without affecting the tone of the passive pickups. You will also be able to use other EMG accessory circuits such as the SPC, RPC, EXB, EXG, etc. For this application, we recommend ordering the PA-2 without the switch for easy installation on the inside of a guitar."
Or, your battery could be almost dead..... Throw in a fresh one and see what happens.
