EQing the stereo signal
Some mastering processors offer multiband stereo width control. This makes sense when the same crossovers also feed compressors, saturators and limiters. But for simple stereo width control, a network of crossover filters is like cracking nuts with a sledgehammer! It’s better to simply EQ the side channel, which is the difference between left and right channels.
You can derive the side channel by subtracting the right channel from the left channel which, in practice, means inverting the polarity of the right channel before adding it to the left. Alternatively, call up one of the free or DAW-bundled plugins that enable extraction of mid or side signals from a regular stereo file.
Altering the side channel’s level will change overall stereo width: turn it up for a wider mix, down for narrower, and mute it to get mono.
Boosting or cutting the side channel with EQ widens or narrows those specific frequency bands. Perhaps the simplest application is a high-pass filter: removing all content below 80-120Hz from the side channel will mono your sub-bass frequencies. This is particularly important if you’re mastering for vinyl.
If you’re cutting low-mids from the mid channel, you might want to compensate by boosting the same region in the side, to put back some of the fatness or warmth it may have lost.
More importantly, wide low-mid frequencies sound awesome – much more impressive than wide treble! A little bit of low-mid boost in the side channel is often a good thing, but be careful not to overdo it.
Likewise, if boosting the high end of the mid channel leaves it sounding too bright in stereo, cut that region back in the side channel to compensate. You can’t hear stereo sub-bass, so many people assume that spatial perception simply improves as the frequency increases, but this really isn’t the case. If the midrange of the mix is nice and wide, the highest frequencies can go right back to mono and no one will care. In fact, narrowing high frequencies can often make the mix seem more focused and coherent, and the midrange seem wider by contrast.