The Quadropolis Surround-Plugin is a VST effect Plugin dedicated to Quadraphonic Music. There was a lot of research on Quadraphonics back in the early 70s.
This Plugin simulates an anechoic chamber with 4 loudspeakers in it (starting from 45° in 90° steps). Within the soundfield there is - of course - the listeners head, which is simulated with a 512 Tap HRTF-Convolver (FIR-Filter). This causes high CPU-Load (about 50% @2GHz, but improvements are on the way).
The plugin gives you the ability to mix your quadraphonic music, without using 4 loudspeakers. Only Headphones are needed. Just put the plugin (with 4 Inputs and 2 Outputs) at the end of the mixing chain and enjoy panning the music around.