I can only relate my experience, but I upgraded from Maggie MGIIIas to 3.7s then to 20.7s after a few months.
I was bi-amping the MGIIIs with 2 Audio Research DH400MK2s. The AR amps can be bridged and will drive the 20.7s but to my ears the sound was not as good as the using the single amp in stereo mode. Perhaps it is a slight increase in noise when using bridged and the now what looks like a 2 ohm load presented to amp. It is a SLIGHT difference, but you can hear a tad bit of additional "noise"
The AR is 200 watts a side at 8ohms and 400 at 4 and plays loud enough in my 20x19 room with vaulted ceilings.
If you are a planer guy, the 20.7s are magnificent, again IMO. The blending of the drivers, the stage, deeper bass all make the upgrade a "non-brainer" assuming space, money etc.
My only issue with Maggies over the years was the fact the tweeter seemed to always call attention to itself. This got better and better but I think as far as a single cohesive speaker, even the 3.7 sounded better in this respect to 20.1Rs.
Get the 20.7s and I doubt you will ever look back or regret.