It isn't that great of a price at $630, when it wasn't much more new. I have the MC700 and I like it, imaging is not an issue. That has more to do with your speakers and source than the Pre, though it does contribute. I had the UMC-200 before it and the MC700 has a better calibration software and does sound better.
No issues with surround processing, it handles Atmos tracks perfectly fine as TrueHD, but you are correct that you cannot add height speakers, something I am interested in doing as well. There's not really another option in their lineup until you get the XMC-2 that has Atmos and the height channels. The XMC-1 was not too big of a step up in price, but it lacks height channels as well. I am considering the XMC-2 but the Marantz 7705 is less and appears to have pretty much the same features.