How come my Windmills are so quiet?

[MatsD]

I perfomed a simple hum-test today. I placed all my basses side by side, and one by one I connected them to my amp and noted the amount of hum.

One thing was obvious; my two Windmill basses were dead quiet while all others emitted considerable amounts of hum. Most of this hum goes away when you hold the basses and play them, i.e. ground them with your body, but the Windmills were totally silent even when not grounding them this way.

All basses are passive and have single coil pickups, except for one of the Windmills which have a single coil neck pickup and a humbucker bridge pickup. But the one with two single coils is just as quiet.

So, does Windmill have a secret way of grounding/shielding their electronics, and what can I do to make my other basses just as quiet?

Any ideas?

/Mats D