The C major add 9 guitar chord sounds fantastic and is simple to play When I found the most common voicing for a Cadd9 chord, I played it over and over. I have 12 guitar chord voicings for the Cadd9 chord and cover the notes, intervals and the 3 major scales where you will find it. I also have a number of popular song examples that use the chord.
The A6 chord, like all 6 chords, is not the most common chords that you'll see in popular music genres. However, you will often see the chord that it equals: F#m7. The A6 chord has the notes A-C#-E-F# and comes from the keys of A, D, & E major. I have 16 open and closed A6 guitar chord shapes and list some popular songs that use the chord.