If one thinks about it a surround receiver is a sum of 4 distinct parts: Tuner, preamplifier, processor, and amplifier.
I think it's pretty obvious what the amplifier and tuner parts do, but I could see how the processor and preamplifier could be confused. The preamplifier controls basically anything in the analog realm including tonal controls, level controls, and input switching.
A processor handles digital processes, such as digital to analog conversions, bitstream processing, post processing (bass management, surround algorithms).
