Starting with the digital section. IC1 is the clock generator, which runs at four times the sampling frequency. The frequency is adjustable over a limited range by VR1, the Delay Fine control. This slightly unusual configuration gives an output with an approximately equal mark-space ratio.
http://www.web-ee.com/Electronic-Projects/projects/digital_echo/index.htm
The Surround Sound Processor (Project 18) is a standard Hafler matrix, and will provide a passable rear channel signal. In order to confuse the brain, we really need to delay the signal, which makes it sound as if it were further away. This principle is used in virtually all commercial units, but they have a tendency to be somewhat more complex than a simple passive unit.
http://sound.westhost.com/project26.htm
Here is an experiment with some old speaker drivers which delivers loooonng delay low frequency DIY reverb for almost no cost.
http://www.electronicpeasant.com/projects/springs/springs.html