I agree with Cleg. The ears normally locate sound by timing differences between ears and the shape of our ears (front to back differences). This is mostly null and void with headphones so most of the time when I read about soundstage all I think is tonality. Quite often if the treble is forward many will say it has a large soundstage and more 'air'. This isn't always the case but it usually correlates to the sound signature of the gear being described. Most of what I hear as soundstage actually has more to do with the recording than the gear it's listened on.
With headphones and source gear I tend to focus on imaging, which is also a false reality with headphones, but at least imaging derived from the timing and decay and L/R balance can get the listener closer to reality. Binaural recordings are exceptional at this. So in the end it really is pshycoacoustics that are at play because it's just an interpretation by our brain of how they are supposed to hear things.
Remember, we don't actually 'hear' the air pressure. We 'hear' electrical impulses in our brain converted from air pressure by our ears and then the brain needs to deal with these impulses to construct a perception we call sound. Just like our brains actually don't see light. The visual cortex is in complete darkness and only interprets impulses from the optical nerve, but no light actually reaches our brain. I find it interesting that we take air pressure, record it as a different medium (magnetic tape, vinyl, digital, etc.) send it as a voltage through a wire and convert it back to air pressure using magnets in the headphone only to be converted back to impulses for our brain to interpret.
To bring it back to the N5, when your gear can delineate the start and stop of a note well and play with little to no audible timing distortion (the brain is exceedingly sensitive to timing errors) then it has an easier time to reconstruct the image of the performance, and therefore the soundstage and imaging in the recording. For its price the N5 is pretty good at this, though there is much better gear out there, but much more expensive as well.