Many event spaces only require a single set of main loudspeakers, but others not so much. Get into a event venue with large, long, deep, or L/T shaped ballrooms and it's a totally different dynamic. This is when you need to consider how your system will interact with the audience. Do you need the main console to feed remote speakers around corners, down hallways or behind doors? Is the music bass intensive requiring subs? Are there restrictive sound ordinances mandating creative loudspeaker placement and output limits? These are all questions in my mental notebook when I visit an event venue ahead of the event itself. Let's assume one or more of these conditions exist. The easiest way to make it all work, and have a centralized means to manage it is with a signal processor. When running multiple pairs of loudspeakers, or even multiple zones, a mobile DJ can benefit from using a signal processor in many ways. In most cases a DJ will have a single set of signal outputs that are daisy chained across however many speakers or amplifiers they use. Tuning this setup requires the powered speakers to have digital signal processing (DSP) builtin, or amplifiers with DSP for passive setups. This is great if the same equipment is used in the same space every time, but when your acoustics and room layout change, as is the case with a mobile DJ, it can be challenging. A better solution is to use a signal processor so that all tuning comes from the console.
Setting all speakers with a flat EQ at 0 db allows independent adjustments from a single location. A professional 6 channel processor allows you to tailor the outputs with gain, pre & post equalization, compression, limiter, delay, and auto-feedback suppression to fit the need.
We use the DBX DriveRack 260 with custom 1x4+2 stereo and mono profiles to tune each speaker set to it's optimal range. The 1x4 is for the mains - stereo or mono, a single mono channel for transmitting a wireless signal to one or more remote speakers, and a single mono channel for the sub-woofers.
In large venues you may need to adjust delay when using speakers that are placed in varying distances from the audience to ensure the sound from all speakers reaches them at the same time. Even a small amount of sound wave differential can lend to audio fatigue without the listeners even knowing it. Another advantage is the ability to define which input is carried through the output. Have you been to an event where your favorite song was playing, only to hear portions of it drop out? This is usually because you are in an area or 'zone' that is only carrying one side of a stereo signal. One solution to this is to feed all inputs with a mono signal, but in my experiences the highs get muddied, and the effect of separation is lost on the dance floor, loosely comparing AM radio to FM Stereo. Our solution is to run the 1x4 in stereo mode (channels 1-4), and the two secondary zones (channels 5 & 6) in mono. You always want to run your subs with a mono signal, and when we transmit the signal to remote speakers they are also fed a mono signal since there can be one or more receiving the signal. On occasion when we are outside and playing to a large area, we do switch to a profile that feeds the 1x4 with a mono signal for uniformity.
On occasion we run across a venue with a house audio setup. More often than not it's because of a noise ordinance or an unrelenting neighbor that calls the police every time the sound reaches 30 db. These systems are usually regulated by a compressor, or even a limiter, that stops or uses an attack curve to suppress anything above certain decibels, and installed by sound technicians, engineers or the like. Both options are components of a professional signal processor, and while it can be the bane of a DJ's existence, it keeps the venue in business.
In the end, it's all about giving the audience the best experience you can while optimizing (and protecting) your equipment, and keeping within guidelines. Get caught up in an over zealous crowd that feeds off of punchy bass and you can find yourself pushing too much. Setting limiters or compressors to act as governors will save your equipment for another day.