While I have minimal interest in this thread as it seems it will degrade shortly. There is a large audio myth that is being perpetrated... Imaging and soundstage are not in anyway based on power the list below are the primary factors that control imaging [in order of magnitude]. At the same time these factors greatly contribute to perceived soundstage.
Room interaction - It has been shown through countless perceptual studies that off-axis response with similar magnitude to the axial response increases listening enjoyment in stereophonic situations. This was found to be in part due to increase realism caused by superior imaging. There are however some exemptions to these findings. Any speaker with designed to have off axis dispersion will have terrible imaging if placed within one foot of boundaries in a highly reverberant room.
Symmetry of response - this refers to the frequency and phase of the loudspeakers and effects the perception of being in the centered. Symmetry of speaker placement and listener position do play a part in this.
Treble response - Referring to a typical two channel system a slight boost in treble (3dB above 6kHz) will yield perceived "tighter" imaging with a smaller sweet spot while the same cut will yield wider, but more "loose" imaging.
You will notice power is not once noted in this highly studied area. This is because of a plain and simple truth: If a speaker is receiving enough power [from a properly designed amplifier for the application] for the dynamics required of it this power will be inaudible. Excess power is pointless plain and simple that is what excess is...
Remember, in a typical home listening environment much of what is listened to is actually the reverberant sound field not just the speakers output. This is why off axis response is so important along with properly treating ones room.
