Watch the Alternate Bass Lines Pt. 2 online guitar lesson by Brad Carlton from Guitar Lab: The Imaj7 IVm6 Progression
In this example, you'll see the principle of modes at work when we play the second chord of our two chord vamp. The Ebm6 is the Im6 in Eb melodic minor - this has been our viewpoint up to now. Mode IV of Eb melodic minor is Ab Lydian b7. Your parent scale won't change when you substitute Ab9(13) for the Ebm6. But now, the Ab note becomes a strong melodic choice. As always, it's critical to know how each note that you play functions against the prevailing chord.