Issue with Conklin bass

I haven't heard that and I've had mine (GT5) for 15y or so. I always thought the PUs were passive but I've honestly never looked. It's getting a resting on the W/E and a full set up, so I'll look then and check.

 

This is my diagnostic process for a situation like this.

0) (assumption is this step has already happened) Go over the wiring with a fine toothed comb and make sure there are no obvious faults - stray shorts, bad looking connections, corrosion, botched repairs from the past, etc. Extra attention on the jack itself. Check continuity with a cable inserted. Wiggle it. Make sure the jack is good. Also make sure the battery clip is good, and there is 9v (more like 9.4+ for a fresh battery) present at the preamp. Jacks and battery clips are natural wear items.

1) Disconnect pickup wires from wherever they go (i.e. a selector switch, blend pot, etc).
2) Disconnect the signal wire from the jack. Your preamp is now removed from the circuit.
3) One at a time, wire each pickup directly to the jack. Test play for a while. See if the issue is present.

If the pickups play fine wired directly to the jack, then I would tend to agree with your tech that the problem is in the preamp. If a pickup does not play well wired directly to the jack - well, it's the pickup.