noisy basses - what's the deal?

I have a bunch of Warwick basses (4 to be exact, 3 active, one passive) and each one of them has a buzz similar to the buzz coming from single-coil pickups...when you turn to the right direction, the buzz goes away... it's worse in some situations than others... I thought this only happened with single-coils. I took the Streamer to my repair guy, wanting to get it shielded, but he replaced the preamp instead (said shielding would not help much). I think it got better, but the buzz is still there. What's the deal? Shouldn't my bass(es) be perfectly quiet? Is it unreasonable to want them to be?