The problem with garageband and modplug are that at the end of the day, they're still nothing more than midi. Modplug comes close, but it stops short of applying acoustics calculations and is severely limited to .mod and .mid standards. Same with Fruityloops (or whatever it's called). They all rely primarily on (though rather good) procedural sample creation, while Reason takes the soundfont concept and pumps steroids into its veins.
Seriously, send a midi my way and I can have a rough cut for you in very little time.