Exactly!!! I reached the same conclusion regarding the Maggie 'bass solution' and purchased a pair of REL Carbon Subs to go with my Maggie 20.7's. Oh Man... a match made in heaven!
Do not try to add bass (or anything) to what's coming from the panels. Do not try to 'blend' the subs with what the panel is already producing. (A lot of people are never happy with subs and Maggie's because they make this mistake.)
Simply set the sub XOver low enough that it only extends into the lowest octave that the Maggie's don't touch! And for optimum performance with Maggie's, rather than placing the subs in the room corners behind the subs (as REL and other Sub manufacturers often recommend), move them into the same plane (or slightly forward to compensate for time alignment), and adjacent to the Maggie bass panel. That's how you 'blend' panels with cones!