Time to open a can of worms :)
One of the reasons we purchase Maggies is for the transparency and neutrality of the music we listen to, getting rid of the inherent sound signature of the box. I’ve not owned smaller panel Maggies, 2.5r and up to 20.1’s. I would imagine there would be a need for a sub for the smaller Maggies, 1.7 and under, not saying in all cases but given the smaller size of the panels they may need that extra bottom end. IMO adding a sub to the larger panels counter acts the whole point of going box-less. Maggie bass will not shake the glasses off the shelves, but they will reproduce the sound of the instruments in the way they were recorded. At the risk of opening another can, Large panel Maggies require tons of good clean power and current and to say my 50 watt tube amp does fine with my 30.1 panels is like saying my Pinto does fine on the Autobahn, the Pinto will get you there, but where is the pleasure of driving the Autobahn. I love large Panel Maggie bass with high powered amps that are capable of driving them. Fast , articulate and natural, that is how I describe Maggie bass. Adding a box defeats the whole point. A high end sub, no matter how much you spend is not going to sound like a Maggie.
Enjoy the Music