I have the smaller 3.6r and they rock in my system. It was not always the case though. Adding the Spectron mono-blocks was a huge improvement. But it was after I got the Joule Electra Marianne Electra preamp that I felt how much slam the Maggies can have.
I think the Maggies 20.1 should be fine in your system, assuming your listening space is not too large. They can certainly handle a lot of power.
I have a Velodyne HGS-15 and it is not fast enough to keep up with the bass driver in the Maggies. Based on my listening habits (mostly jazz and classical, occasional rock), I don't really have a need for it. So right now the Velodyne is a nice, glossy side table.
Best,
iSanchez