Chances are if the fuse has blown, there are failed electronic components that caused the failure.

Here is a service manual that will be CLOSE to your unit, though not exact:

http://elektrotanya.com/mackie_srm450_active_speaker.rar/download.html

Scroll down to the "Get Manual" on the site and click on the link. The file is compressed. The first part is the schematic, the second the assembly and parts list. Likely the main low freq power amp has shorted transistors.