Millercarbon is right about the EXACT angle necessary to position speakers.... That is mandatory...While this certainly helps, I've never had to do that to get excellent imaging and depth. But I work with speakers that aren't beamy on the top end. If you have issues with beaminess, there will be a small sweet spot and speaker position will be critical.
The other issue I find that seems to affect depth and soundstage width is the electronics. If you have phase shift issues in the upper region of the audio band, you'll lose soundstage information as a result. Phase shift is usually caused by a rolloff that is often outside the audio band. In most amplifiers, the upper limit where the -3dB point is encountered in the amplifier's frequency response is where this phase shift starts, and it has influence down to 1/10th the cutoff frequency. So if your amplifier rolls off at 50KHz you can expect effects down to 5KHz.
Also, if your electronics has a distortion character that increases with frequency (which describes most solid state amplifiers made in the last 70 years) maximizing the soundstage will be more difficult, since you will have distortion at the higher frequencies masking low level detail that is part of the soundstage presentation (this distortion characteristic is also responsible for brightness and harshness). If you have wondered why some designers eschew negative feedback, this is why, since the application of feedback is tricky. If you have some but not enough you'll get that rising distortion with frequency thing I just mentioned. 'Enough' is usually about 35dB or more! Semiconductors that allow for the sort of gain bandwidth product (which is to feedback like gasoline is to a car) really haven't existed for most of the last 70 years.
To avoid these issues the electronics either has to have so much feedback that it doesn't make more distortion at higher frequencies and phase shift in the audio band, or you run no feedback at all and simply have a wide bandwidth amplifier. Now of course many reading this will be thinking that they are getting fine soundstaging with their older solid state gear (which almost by definition is what I've been talking about); So to be clear here I am talking about **maximizing** the ability to reproduce the soundstage correctly. You certainly can get good soundstage effects with older gear.