With a clamp on meter at the service what is the amperage draw per leg?
Assuming there is a main, what is the draw with the main breaker off?
I assume you did the above when isolating the subpanel anyway. What about further isolating the things to individual circuits in the subpanel? What about turning off the breakers in the sub and monitoring the current in the feeder to the subpanel?
I would suspect a high resistant ground fault drawing about 15-20 amps somewhere.
