I think of the chord in Stairway to heaven as D/F#. An inversion, not a minor 6th. I think when chris said the minor 6th chord can be used as a tonic, he made the point why we have a minor 6th chord. Melodic minor sequences for one, and by the way that does make sense chris. Thanx. Also on a ii chord, especially when you don't want to sub a dominant or half diminished. I think of minor 6th chords as being a homebase for dorian melodies, take the chord progression ii-iii-IV-iii. The "ii" is nice as a minor 6th chord and I wouldn't second guess myself into thinking it was an inversion or sub.