The 20.1 is the only Maggie with a Quasi-Ribbon midrange.
The 3.6 has a planar midrange like the older 3.5 & 3.3.
I've been waiting, (patiently!) since I sold my 3.3rs for a 3 series Maggie w/a Q.R. midrange. The Absolute Sound is the only magazine I've seen that detailed the performance jump when the 20.1 was auditioned with the Q.R. midrange, (I don't remember the issue #, sorry!)
I own Apogee Mini-Grands now & they fill the bill (for me)quite nicely!
The Magnepan website is pretty vague on this, maybe your nearest dealer can help....