Main-in is a direct connection to the amp section in a receiver - think of it as a way to treat the receiver as a power amp, with nothing (volume control, etc.) in the path. The pre-amp outputs are typically jumpered to the main-ins for normal operation, and if necessary it is possible to put an EQ or other bit of processing in between the pre-outs and the main-ins. The 7.1 analog input goes through the volume control and some other portions of the receiver (possibly bass management, time delay, etc.).