Hi. I think it always a good idea to answer the person who is posting with a correct and relevant answer. Otherwise, we are not helping, which which is what this site is supposed to do. Once we do that, we can layer in other information.
I have heard Magnepans with CJ amps (MF2250SE, Premier 350, Classic 120SE, Premier 140, and LP 270SEM). For that matter I have heard them with VAC, Herron, Pass, and even a Fisher SA 1000.
As a disclaimer, I am a former speaker manufacturer and now provide concierge service for several companies, helping people build systems.
First the CJ MF 2550SE and Premier 350 may be two of the best amps CJ has ever made and may be two of the best sounding SS amps ever made. The standard MF2550 is a slight bit less detailed than the SE version, but still a lovely, musical amp. I could never use the words used above to describe it. Those words simply do not fit and mean that there was something else at play.
Read the reviews on the MF2550SE, there is a good comparison to the 350. The 350 simply engages you at a different level than the 2250SE. The 350 has power to spare and is a hair warmer than the 2250SE so it would mate well with the Magnepans. Speaker wire will matter.
I have a customer with Manegpan 20.7 who uses a CJ MF2275SE and it is a lovely system. I can list to it for hours. The 2275SE is a hair less detailed than the 2250SE. But as I said, we are "splitting hairs."
Yes, I think the 350 would work well with the 20.7s. Sorry for the long reply. Enjoy!