This is followed by the chords Eb7/Bb to D7/A, both with the 5th in the bass, which creates a warm sound. D7/A is the VI (six) chord, and I approached it from a half step, or one fret, above.
As the V chord, the A is the most likely candidate in our sights for substitution. Let’s make it a dominant 7th, then shift it up a tritone (#4/b5) to give us Eb7. Then let’s go even further ...