"Front" in pre-amp mode does not include the C(enter), but only L+R channels (RZ50 & RZ70).
DLBC is, if I remember it correctly, als available now for the RZ50 (RZ70 too) at a lower price for multiple subwoofers. "Full" license covers only regular bass management.
In pre-amp mode the power amps are never switched off, but only run in low power (idle) mode depending on the power amps efficency.
They are just excluded from handling the signal.