Fellow Audiophiles............I am building a new home and setting up a first surround sound system. I have 4 Sonance MAG6R speakers to use for left/right and surround. My question is, would a 600 or a 650 be a better pairing with the Sonance as a center channel. I need to go with an in-ceiling speaker since the wife doesn't want a sound bar like speaker sitting on our nice cabinet.

Any help would be greatly appreciated.

For an in-ceiling center channel application my first recommendation would be the HTR-7000. HTR-7000 was initially designed as an in-ceiling L/C/R for exactly this application. The fixed angle of the driver and tweeter will help drop voices and dialog closer to the point of the screen and you won't have as much of that "in-ceiling" effect.
If this won't fit the application for some reason, I would go with the 650.

Hope this helps, good luck and enjoy!