1. A pre-amp generally does the processing and selection of inputs. You can consider it to be equivalent to a receiver without a tuner and the final amplification stage.
2. A good sub is far more likely to make your system sound better.
3. You would buy an amp with the number of channels you would like, or multiples thereof (in other words, 5 mono amps for your existing speakers, or 2 stereo and one mono, or a 5 channel, or a 7 channel...)
I am the Doctor, and THIS... is my SPOON!