This article is based on something so fundamental, it is as much about composition and arrangement as it is about mixing. One of the hardest parts of a track to mix is the bass and kick, this poses a problem when you are making house and other electronic genres which rely on both being at the forefront. When badly mixed, the clash of the two means that any compression will shatter the weight that they should have. It all comes down to the fact that they both occupy the same frequency space. Constructive interference in particular, can cause volume spikes which trigger a compressor more than it should and the result is a highly attenuated signal which can kill impact.
While there are techniques such as side-chain compression, which means that the kick can sit nicely on top of the bass, it involves using effects and has a noticeable (yet sometimes desirable) effect on the sound. For me, the number 1 rule of mixing is that a song shouldn't need much mixing in the first place. Your track should be arranged and suitably composed such that everything fits together in the first place, with mixing just ironing out the issues.
Just as when baking a cake, it is better to know the exact ingredients and quantities beforehand, as opposed to tasting the batter and adding things to taste (too much of this can ruin the cake), mixing is the same. Classical composers, as another example, had to work hard to make sure that the songs mixed well through both instrumentation and melody choice, without any of modern day mixing equipment, this is essentially what we are going to do today.
You don't need to look far to find where the issue lies, we can identify what is wrong when the bass and kick don't blend well together. Ultimately it can be traced to one simple fact: they often don't blend well together. As simple as it sounds, by playing about with the melody of your bass-line, you can completely sidestep the issue. Try placing your bass notes in between the kicks, the most notable example would be the genre Melbourne Bounce, where the bass plays on every off beat. This ensures that they never occur at the same time and mixing the two is a case of ensuring that they are in key, and their tails blend well together, which is much easier.
While some may disagree, I regard most Melbourne Bounce tracks as pretty horrendous and as a whole, quite un-inventive. This tutorial is not intending to tell you to drop your genre and switch to Melbourne Bounce, however, we can draw some inspiration from it.
Instead of just whacking a bass note on the off beat, we can play about. We can have the main bass notes accentuated on the offbeats and syncopated timings, then even allow a few to hit alongside the kick. One of the best examples of this is LO99's track "Take Me Back":
This song has a really solid kick, and a very UK-style house bass sound. The main impact of the drop is the first two kick beats with the bass hitting on each offbeat. After that they come together, by now the impact has already been made. I have created the rough rhythmic notation below if you are interested (notes may be wrong as it's a rough sketch).
Another great way to allow better coherence between kick and bass is to decide which one "owns" those tricky frequencies. If for example, you have a really nice thumpy kick with a heavy-weight low end, you probably don't need to worry as much about the sub frequency content of the bass synth. Conversely, if you have a very sub-orientated bassline, you can take some of the weight out of the kick. The track below by Malaa has both playing simultaneously for the kick drum beats, but the bass fills most of the sub frequencies, this works because the kick drum used is short, and relatively thin compared to many other house tracks.
Finally, again under instrumentation, a great trick, especially for heavier bass sounds is to separate the components. Layer a sub bass sound with a heavier mid-range sound. The mid-range bass can play throughout the kick hit, and the sub can either be side-chained against the kick so it provides room, or the attack can be increased on the synth's amp envelope setting, so it hits full volume after the kick has played.
I am going to use Skrillex's Kyoto as an example because the sub audibly reaches peak volume after the kick has hit, the version I have provided in this is the bass boosted video as it is much easier to hear over the heavy mid frequencies.
This list is of course not exhaustive and mixing skill remains to be a large factor in the eventual sound quality, but before you slam a multiband compressor on a track to try and fix those frequencies, you may want to remove the issue at the source to save over-saturating the track with effects. Mixing should be as much about bringing out the best bits, as it is carving out parts of the sounds you so lovingly made, just so they fit together.