I think the problem you have here is that there's a ground connection between the bridge and the common ground in the control cavity that's not connected. You might have missed this when reassembling the electronics.
Alembics shouldn't need this as much as something like a traditional Jazz Bass, but it's common for the bridge and strings to be connected to the electrical ground. When connected this way, when you touch the strings, your body becomes part of the ground plane as well. Since you're a big bag of non-conductive water (no offense intended), your body makes a fairly good shield.
There's probably a wire that runs from the conductive paint or the EQ board to the base of the bridge that makes you part of the circuit. If you left either end of that wire disconnected, you'll get buzz like you describe. When you touch the shell of your cable, your hand is making the ground connection and you're part of the shield. When somebody else touches the cable they're part of the shielding now, but they're not close enough to the pickups to held reduce the noise.
The interference you're picking up has some high-frequency components. That's why turning down the tone control reduces the level of the noise - it's just filtering the noise out along with your instrument's treble.
The reason that this is a little odd is that your Spoiler has hum-cancelling pickups, so it shouldn't be all that noisy (vs. something like a Strat that has true single-coil pickups). The Alembic folks can confirm whether there's supposed to be a bridge ground wire.
The grounded bridge helps noise, but you do need to be a bit careful. If any of the outlets on stage are wired backward, if there are any 2-prong AC cords, or if there are any 3-prong to 2-prong adapters, then there's a shock hazard with the grounded bridge. For instrument amplifiers, signal ground also equals electrical ground and if your polarity is reversed with regard to anything else on stage, the current will flow though YOU. That'w unpleasant if you're in the US and could be fatal if you're in Europe with 230VAC mains.
David Fung
