is CA 840a an adequate amp for magnepan mmg/mg12?

I wouldn't think you would have much of a problem with the MMGs. I used a Linn Classik for a couple of years followed by a 30 watt tube amp for the next 8 years with no issues. That said, I have a small listening room. Trying to drive them in a large room may cause issues without a more substantial amp.

Don't know the particular amp, but I own the MMG speakers. Unless there's something strange going on, 200 watts into 4 ohms should be fine. More power might very well be better (especially for high SPL and/or a large room), but you're definitely in the safe range.

Good Luck

Marty