I used the 650Ms with my thiel 7.2s for a few years. They are very good amps, but in comparison to the 700cx i think they can sound a little dark. I preferred this, but many people dont. Both very good amps, win/win choice.
Your krell has more than enough power for the Sophia. Only that Krell's own preamp. can bring out the full potential of the 700cx - or any of the FPB series. Cary, being may give the illusion of sweeter sound, but the sky brightens with a KCT or the newer version such as 202