on re-reading the op, how is the cone coming undone? and now that it has been re-tightened has it come undone again? u must make sure all the nuts and washers and spacers on the axle are tight when the wheel is bolted up. if there is any gap that may allow the cone to undo, but it has to have some 'air' to undo into.
spacing the hub will only help the CL and would have no effect on the cones undoing. If LMNH has done it up correctly and the frame is tight to the Axle and there is no dead air between any of the other components then you should not have a repeat of this.
on re-reading the op, how is the cone coming undone? and now that it has been re-tightened has it come undone again? u must make sure all the nuts and washers and spacers on the axle are tight when the wheel is bolted up. if there is any gap that may allow the cone to undo, but it has to have some 'air' to undo into.
spacing the hub will only help the CL and would have no effect on the cones undoing. If LMNH has done it up correctly and the frame is tight to the Axle and there is no dead air between any of the other components then you should not have a repeat of this.