The iCans 3D settings don't really do much for gaming.
While it is a small improvement over plain stereo, it's not the same thing as actual virtual surround coming from a 5.1 source. It can improve sound separation and change the soundstage characteristics, but it doesn't create accurate positioning from a 2 channel source or anything.
As for stacking it on top of Dolby Headphone, it sounds interesting at times and can make things sound even more out-of-head, but it also clashes with it a bit. It can sometimes screw with the positional accuracy, so I would just leave it off for that. When I get around to recording demos of the iCan I can try and include a Dolby Headphone vs Dolby Headphone + iCan 3D effect demo.
If your talking about the iCan's headphone jacks output impedance, it's <0.5 Ω ohms - so extremely low (a good thing).