Hi,
I did some interesting experiments with my guitar
and my home theatre system (5 sattelite speakers + sub).
I found that we need in special tunings and presets
to have a good guitar sound from 5.1 systems.
The GuitarFX tunings for 5.1 is different from presets
for stereo headphones or Hi-Fi stereo speakers.
If you have 5.1 and want to try to it for guitar fx
you can download this preset from http://GuitarFX.Net
Preset Number 21288 (see bottom of http://GuitarFX.Net)

Dmitry.