The only electronics packages with hum cancelling trimmers are the Series I and II (your 76 SI), since as Graeme said, the others don't need it. And you are right, the Epic package only has one gain trimmer, while your other Alembics all have two, one for each pickup. The thin MXY pickup actually sounds no different from an AXY pickup; the only difference is the size of the shell. The character may be the contribution of the Maple top, or just the overall combination of woods. And as Graeme mentioned, the hiss is probably something other than the pickups. Check inside the control cavity to see if there is a ferrite bead. The pickup leads should be looped through it. Compare with your other non-Series basses. While your in there, glace at the shielding to make sure it's all still there and that none of the componants are touching it. Make sure the four knobs and the 1/4 jack are snug. The electronics are grounded by being fastened securely to the top shielding.
