Probably the most important thing that separates pro-sounding mixes and amateur ones is the low-end. When you hear a mix that's done by the big guys, it always has a solid and driving low-end that outlines the chord changes and makes you “feel” the music in your chest.
There’s this low-end sweet-spot that is fat enough to drive your speakers and deliver the energy, but not too boomy and resonant so that it overpowers everything else, turning the mix into a muddy mess.
Here are a couple of quick tips to help you improve your low-end when mixing.
RESOLVING TUNING ISSUES
This is where a lot of mixes fall apart - inconsistent tuning of the instruments. Whether it's the instruments being slightly out of tune with each other or issues caused by the instruments not being properly intonated which makes them out of tune with themselves. Sometimes even a perfectly tuned instrument can sound horrible because of bad playing technique.
These issues usually are found in guitar tracks - acoustic, electric and bass, as their necks are almost impossible to intonate perfectly, without implementing special fretwork or using a multi-scale design.
There are some things you can do to improve the low-end stability - first of all you can go through the bass tracks using a pitch correction tool, like Melodyne or auto-tune (other tools will also work) and tune the notes to the center of the pitch. This will make the bass “gel” much better with other elements of your mix, such as keyboards and guitars.
If a bass track is especially hard to fit into a mix, you can actually program the low end using a virtual bass instrument and use it alongside the real track in a multi-band fashion. If you want to use this trick, you need to divide each track into specific frequency bands, so that the programmed bass lives in the lows, creating a solid foundation and the real track lives in the mids and highs, adding realism to the performance. It's a little tedious, but it can really save a mix.
FIXING POLARITY PROBLEMS
This one's a little harder to catch, but it can make a massive difference. Polarity problems can absolutely destroy a mix, especially your drum tracks, as it becomes very important when you have multiple mics used on the same instrument.
If you are mixing kick drums, toms or bass guitar that's recorded with more than one microphone, just go through the waveforms and inspect them for opposite polarity.
If the visuals are not clear enough, just flip the polarity switch with the multiple tracks playing and listen to the results - if the sound becomes thin and weird, the polarity was correct. If the sound suddenly becomes louder and fuller, you've eliminated a polarity problem.
It goes beyond that - rock music often has lower tuned guitars that partly occupy the same frequencies as the bass guitar. If those two instruments are out of polarity, the results will be less than optimal. The same goes for kick and bass relationships - make sure that one is not cancelling out a part of the other.
There is also the concept of absolute polarity - a mix that has all instruments pushing the speaker cone rather than some of them pulling it back, will sound more clear, full and powerful. The main idea is to work with your speakers, not against them to utilize their full potential and to minimize instruments cancelling each other out.
SMART EQ MOVES
Using a thought-out and tactical EQ approach can make a world of difference - it can make every instrument have its own space in the mix, which will provide more separation, width and clarity.
This is ever so important when mixing low-end elements. First you should determine where each element of the low end should go - the main elements here being kick and bass. Usually the most powerful low-end can be achieved when one lives slightly lower than the other, so that they aren't fighting with each other.
The main spots are around 60 Hz and around 80 Hz - of course, these can vary slightly, depending on the instruments used, their tuning and the key of the song. In modern rock usually you would have the fundamental of the kick drum living at around 60 Hz and the bass guitar at around 80 Hz. Pop and dance music is usually the exact opposite with the kick being placed higher than the bass. Whichever way you choose to go, be sure to use high-pass filters to make room for each of those elements.
When mixing the low-end elements, keep in mind which instruments you don't want occupying this frequency space at all, like, for example guitars that usually have cabinets booming away in the low frequencies. Synths are also a huge headroom hog that can easily cloud the low-end. Use filters to carve out the unnecessary frequencies. Be careful though - steep high-pass curves (higher than 6 dB per octave) can introduce phase shifting.
If you want to compete with the big sounding commercial mixes, you should reference your work against them to see where you stand. There is no shame in referencing, as it can be your guiding light when you are in doubt or when you need a quick refresh for your ears.
Pick a few mixes that are in the ballpark of what you're mixing - three would be just enough. The reason why you might need more than one reference mix is that you might have tones that are different from the one mix you're referencing to - having a collection of mixes to reference to will help you get a better overall feel of the frequency balance.
If you have an EQ with a spectrum analyzer, you can also use that to create snapshots of the reference tracks which you then can use to visually compare the frequency response of your mixes to. This will also give you a good overall impression of how close you are, especially in the regions you may have a hard time hearing correctly, such as low end.
Mixing low-end is a tactical task and you need to have a plan. Once you've checked all the technical aspects of the tracks, you just need to organize everything in a tidy way to achieve a clear and punchy low-end that will win over clients from other mixers in no time!