The Steadfast (and by extension the NEW Stubborn rule) says, "Steadfast units can always take Break tests on their unmodified Leadership (or the General's unmodified Leadership if the testing unit is in range of his Inspiring Presence special rule - see page 107). "
And just for completeness, the new Stubborn rule reads, "Stubborn units are always steadfast, whether or not they have more ranks than their enemy (see page 54 for details)."
Therefore a unit with Ld 6 which is steadfast and in range of its Ld 10 general would roll on an unmodified 10 when they lose the combat.