The Quark and 8XL should be a good combo in a maxx. If it were way overgeared, I would imagine that the esc would have protested in someway.

It sounds to me like you had a battery failure, probably due to a short someplace.

The motor leads on the Quark are exposed so something could have slid in there and caused the short. Have you tried it again with a fresh set of batteries?

It takes a lot of heat to desolder a connection. What was it that leaked? Are you saying that the batteries leaked, or did the solder flow off of the joints?