Stereo Spread is typically used when mastering. There are several ways to extend the stereo base (or perception of space), including use of reverbs or other effects and altering the signal’s phase. These options can all sound great, but may also weaken the overall sound of your mix by ruining transient responses, for example.
Stereo Spread extends the stereo base by distributing a selectable number of frequency bands from the middle frequency range to the left and right channels. This is done alternately—middle frequencies to the left channel, middle frequencies to the right channel, and so on. This greatly increases the perception of stereo width without making the sound totally unnatural, especially when used on mono recordings.
Note: When setting the Lower and Upper Int. sliders, be aware that the stereo effect is most apparent in the middle and higher frequencies, so distributing low frequencies between the left and right speakers can significantly alter the energy of the overall mix. For this reason, use low values for the Lower Int. parameter, and avoid setting the Lower Freq. parameter below 300 Hz.