Sound stage is the recreation of the recording of a musical event in the x, y, and z axis. (w, d, h) be it in mono/stereo. The x axis is the easiest to reproduce, the z hardest. In large home systems you are really entering deeply into diminishing returns to get that height. Best recordings to identify sound stage easily is to choose a well recorded orchestra such CSO, LPO etc. and see where the wind, strings etc. sections and see if you can identify the section according to the arrangement of that orchestra. Imaging is whats placed within the sound stage. Here you should be able to pick up the individual characteristics of the said instrument, its size in relation to the others in the sound stage. ie a piano should sound much larger than a violin etc.