Put a big speaker in a small room, too much bass. Put a small speaker in big room, too little bass. In general, it is true. There is no one-size-fits-all. In an ideal world, the bass diaphragm would be like an accordion door which could be pulled out and expanded to exactly fit the needs of your acoustical environment. Adding one or more of the Maggie Bass Panels accomplishes the same results as pulling out the bass section of your existing Maggie to a larger size---as if it were like an accordion.
Or think of it like the difference between a 12-inch and a 15-inch dynamic woofer. What does the extra 3 inch diameter of the larger woofer accomplish? Accoustically, the Maggie Bass panel is the same as the extra 3 inch diameter of cone woofer. It increase the total bass diaphragm area of your existing Magneplanars. Yes, the Bass Panel provides deeper bass by getting a "bigger bite on the air", but the Maggie Bass Panel is all about quality bass, not quantity. Getting a little deeper bass is nice, but getting the bass smooth and deep is better.