There are speakers whose soundstage starts on a plane in front of the plane of the drivers, some whose soundstage starts coincident with the plane of the drivers, and some that start behind. All other things being equal, the third category will give you the "deepest" soundstage, but only because it already starts further back to begin with.
One test that's fairly easy is to see whether with a good recording the farthest back point of the soundstage is perceived as lying beyond the front wall (behind the speakers).
Personally, I'm not fond of speakers in category 1; I find them too "in your face". YMMV.