Subs and Magnepans are are subjective taste, and YMMV.
I prefer the 6 footers over the smaller versions, in a big way, after listening to the MG IIa, IIIa, and 3.6 every day, for 40 years, in 8 different rooms. I know what I like. Bigger panels give more resolution, and better, fuller, tone across, a wider frequency range. You do not need to go below 30 Hz, due to room standing waves, with peaks and nulls, that are problematic, are hard to deal with IMO. There is little musical info down that low, however some folks like to feel bass, and want to ruffle pants legs. I do not!
The construction of the panels is well thought out. If you read the patents the bass/mid panels are "tied down" at differing spots, from top to bottom, smoothing the lower bass and mid bass response. Thus putting drums lower than vocals, for realistic vertical sound staging. But the very low bass is projected the full height of the speaker.
Subs may go lower, but are shorter, and do not give the, low frequency full height bass, like running without subs. There is a trade off.
