808 Bass

A tried and tested technique for super heavy bass is to detune an 808 (or 909) bass drum and use distortion to bring out a really distinct and aggressive tone.
The default sound that loads with Rob Papen’s SubBoomBass does a lot of that prep work for you, and with a few tweaks is a really nice sounding percussive bass. You can also make a similar sounding bass in pretty much any soft synth, modulate the pitch of a sine wave with an envelope, using a fast attack, short decay, no sustain, short release. Then use either the synths in-built distortion or a distortion plugin to add drive, and then shape further using the amplitude envelope.

Sub Bypass

Massive has a really nice hack for adding a clean sub to an otherwise heavily modulated or processed sound. Using the Routing section, you can send an oscillator directly to the output, bypassing any filters, Insert FX or Master FX. Useful if you have some heavy distortion on your other oscillators, that you want to keep free from the sub. To achieve this, turn down the Amp of your sub oscillator and head to the Routing tab, highlight the B next to your sub oscillator and connect it to the EQ section. Then turn up the Bypass level (found next to the Master Volume).

EQ

Anything below 30Hz is rumble, and although chances are you won’t hear this on your home studio monitors it’s still advisable to cut from this frequency with a relatively steep low cut EQ.
At the same time, you can use a low pass/high cut filter to roll off any unnecessary high frequencies on your sub. If you’re using a sub with some drive, don’t cut down too low as there may be some nice harmonics on the sound, however a pure sine wave from a soft synth you could cut right down to just after where your sub peaks, which will usually be around 80-100Hz.
If you’re using a Sidechain on your sub, this can cause clicks, due to the fast attack and release times you’ve set on the sidechain compressor. If this is the case, make sure you place the high cut filter after your sidechain to remove the click.

Wave Conscious

Although most subs are made with Sine waves, don’t dismiss using a Triangle wave, which will give a slightly different tone. Massive even has a Triangle/Sine wavetable, where you can set a blend of both. Synth’s such as Serum have a number of different sine waves to choose from, experiment with these to achieve the vibe you’re after.
A neat time-saving trick for Logic users, load the EXS24 sampler, in its default state will load with a sine wave, all you need to do is tweak your envelope and pitch it down, and your sub layer is ready within seconds.

Analog Subs

For a really warm sublayer, try using a rompler where the sounds will have been taken from an analogue synth. Spectrasonics’ bass monster Trilian has a choice selection of these. Sounds taken from such iconic hardware synths will have a really warm sound, and furthermore will have been processed through analog circuitry, fattening the sound further.