Hi there, I have a relatively small HT room. 13'x16'x8'h.

On one 16' side I have a 8"x10" duct that I will need to place a soffit around. Asthetically I tend to want to run the soffit around the entire room. Other than keeping things symetrical does this provide me some bass trapping?

It can provide bass trapping, and also wire runs.

Also, if you are going with the whole sound isolation double drywall and GG in between, you don't want to poke holes throught the drywall for can lights and stuff like that. If you put up the soffit after drywalling, and only poke the electrical through, you can put can lights in the soffit, this will help to keep the sound integrity intact.