Mono the low end.
Once I try this I will never stop using it. This is very useful method especially for mastering electronic and contemporary music that is often played on different sound systems in different clubs. The biggest problem when your track is played on different devices or setups is with the low end.
First of all, almost every sub speaker in clubs is feed by mono out from the mixer, with other words, it plays everything under 62 in mono. But it will sum your base, kick and also the frequencies generated from reverbs, distortions, harmonics from compression and modulations. It will make a mess from your low end. Your good track that sounds perfect in monitors, even on a computer speakers, will be perceived as rubbish.
This is very important when you are doing mastering, especially for electronic music, to have tight and stable low end in every PA and Club.
I will share my way of doing this.
I have this in mind even when I am mixing, but let us presume that I am doing mastering only.
I am making mono up to 100 – 160 Hz depends from the fundamental frequency of the kick. In that range is the fundamental of the kick and the bass and it covers also the second octave of their foundation frequency. But do not forget, always do a low cut on the side. In the Side low end, there are only reflections generated from the reverbs used in the mix, some lower octave harmonic induced from saturation or compression. We don’t want that to be a part of the low end.
Before I Mono the low end I will check the low end on the side, and I will stop with low cut on the frequency that start meaning something to me, I will let on the side only important information in the low end. Usually I am cutting up to 100Hz almost always.
Mono the low end, and you will see the difference and clarity in it. Even if you have only stereo mix for mastering, this will help a lot. Usually I am using Bx_digital V2 from Brainworx, it is a great M/S EQ with mono maker.