For all the production tricks you can use to fatten up your bass, nothing beats actually starting with a really big bass sound. This is easier to achieve with synth basses than electric ones since you can go for pure sub, sine and other wave types and bypass the mid and top end that you get with stringed basses. If you’re lucky enough to have access to real or virtual Moog synths or similar models, you’ll be able to create some awesome fat bass patches. The only thing to watch with these “pure” bass tones is that they don’t actually swamp your mix, being so large. Careful compression and EQ will be required.
Bass is traditionally a mono instrument though obviously since the creation of stereo synths, workstations and of course plug-ins, it’s common to find stereo bass sounds. This isn’t a problem in itself as long as you keep the left and right bass channels centre panned in your mixer. Bass starts to lose its focus, punch and weight as soon as it moves off the centre axis of the stereo field so it’s vital to keep it at the core of your sound. A spectral analyser can be very helpful in showing you visually where the energy lives inside your signal.
Monitors should of course always be optimally positioned in terms of height and angle to give you an accurate reproduction. But when it comes to bass, speakers placed on a flat surface can lose a lot of energy and focus through that surface. Placing speakers onto isolation platforms will have a significant effect on tightening up the bass reproduction. This means it will be more accurate, and you can mix it bigger, knowing that what you are hearing is accurate.
You compress most everything in a project to some extent, but pay particular attention to the bass. With a relatively aggressive compression setup you can keep it loud but contained, without swamping other elements of the track. A multi band compressor can sometimes be helpful, or in the case of electronic bass that has low, mid and even even high parts during a track, sidechaining the compressor can be a good way to ensure it doesn’t suddenly leap out of the mix at inopportune moments.
It’s true that you should’t look to “fix” part of a mix during mastering but the mastering stage is nonetheless one where you can enhance things if you like. A multi band EQ across your master should give you the opportunity to widen, boost or otherwise tweak the bass end a little without affecting the rest of the track. Suites like iZotope’s Ozone give you Q-enabled EQ points which mean you can precisely control the curve of the EQ and change which frequencies it affects. It’s well worth learning about how this works as it can make all the difference to your finished product.