Mono blocks are mono power amplifiers. You would need preamp outputs because if you were going to use an outboard amplifier such as mono blocks you need to be able to feed it the proper signal which is output from said preamp outputs. Going this route allows you to get a cheaper reciever that wont have as good of an amplifier section but will still allow you to have the features that come with a reciever. Because you would be powering the speakers (the M80's at least) with a better, beefier amplifier that can handle their 4ohm load.