Mister Speaker posted:

I mean... yeah, you're not wrong, that's basically a classic reese. But I think the term has evolved and these days if you use it people expect to hear something a lot more grimy, with twisty filter movement and loads of top end.

OP: Get a synthesizer - any basic additive/subtractive with three oscillators will do (you can do it with two but it's better with three). Set one osc to a sine wave and tune it down an octave - that's your sub bass, the real weight of the sound. Where the 'rumble' part comes in is with detuning. Set your other two oscillators to triangle waves (you can even use saws if you want a more intense rumble, since most of their harmonic structure is going to be removed anyway), and detune them finely in opposite directions - i.e: if one of them is tuned sharp by +4 cents, flatten the other one by -4 cents. This detuning creates a beating effect that changes frequency with the note being played. Slap a lowpass filter on the whole thing, with a fairly sharp curve, and bring down the cutoff until you're satisfied. Add a bit of saturation and you're basically done.

The two-oscillator approach is just the pair of detuned triangles, because that's this sound's major component. You can play them as a sub but honestly that beating, that 'rumble' kind of lives a bit higher and you don't necessarily want it in your <80Hz range. Especially if the patch is stereo (which, honestly, sounds great). Make up the sub with some sort of subharmonic generator plugin like Waves MaxxBass, and make sure to sum everything below 100Hz to mono - or don't, if you're doing some really weird ambient poo poo.