Hi Joe,
I owned the Magnepan 3.7 and Apogee Duetta Signature. I also have auditioned the Quad 2912, Magnepan 20.7 and Analysis Audio Omega. The latter three of these speakers are all outstanding and each bring their own special qualities to the table.
I would have to say my favorite of the three are the Magnepan 20.7. These speakers throw a huge soundstage with a massive wall of sound that can extend well beyond the dimensions of the room in width, height and depth. They are also one of the only planar speakers that provides truly magnificent bass, which is fast, tight, deep and can move you. Midrange and treble are seamless and they absolutely excel on piano and vocals, although they can rock too. They are "you are in the venue" speakers. These speakers need a large room that is wide, high and deep enough to let the sound out from behind them. They also need a lot of power to sound their best.
Comparatively, the Quad 2912 produces a more intimate presentation. They are more "the musicians are in your listening room" speakers. They are absolutely gorgeous in the midrange frequencies, probably the best of the three. Piano, vocals and chamber music are magical on these speakers. However, bass is not nearly as tight, fast or robust as the 20.7, nor is treble as extended. They also don't handle as much power as the 20.7 and need to be treated with more care. However, you can run them more effectively with a lower powered amp and in a smaller room than the 20.7.
The Analysis Audio Omega is more similar to the 20.7 than the 2912. They also have that "you are there" presentation. In the treble and midrange, I would say they are slightly better than the 20.7 because they are a little smoother and more organic. However, their soundstage, while very nice, is not nearly as large or impressive as the 20.7. In addition, their bass, while very fast, is not nearly as deep, robust or impactful as the 20.7. I feel these speakers will also perform better with a lower powered amp and in a smaller room than the 20.7. However, they can rock and do have the capacity to handle a higher powered amp and fill a large room as well.
To conclude, I could be extremely happy with any of these speakers. But the 20.7 are my favorite because they have no weak points. They may not be the best in every category but they are close. And they are one of the best values in high-end audio at their price point.
Please keep in my that these are just my personal observations and your mileage could definitely vary.
Best,
Ken