The Sub-Octave: Where "Deep" Comes From
Short answer: A sound is deep when it has real energy at the low end. You can't add that afterwards by muffling the top. The simplest way to put it there is to play a second, quieter copy of the note one octave lower. You still hear one note. It just has weight.
Dark is not the same as deep
I mixed these up for a long time. When a sound was too sharp, I turned the top end down with a filter. That made it darker, like talking through a pillow. It did not make it deeper. Wrap a trumpet in a blanket and you get a muffled trumpet. You don't get a tuba.
A filter can only take away what is already in the sound. If there was never any low energy in it, nothing you remove will put some there.
What an octave is
The same note, twice as high or twice as low. Sing "do" at the bottom of your voice and again at the top. It is the same note on another floor. Because it is the same note, two of them an octave apart blend into one.
The trick
Take a note. Add a second copy one octave below it, much quieter. The ear fuses the two. What you notice is the upper note, as before, but now it has something under it, the way a large dog's bark has more chest than a small dog's.
I added this to the sounds in my Space world after they kept coming out thin and sharp. Nothing got louder and nothing got duller. The sharpness went, because it had been the sound of a note with nothing beneath it.
Don't overdo it
Too much low end is mud. Low sound carries a lot of force, and it covers everything just above it. The layer should be quiet enough that you would not notice it by itself, and you should miss it when it is taken away.
It also needs speakers that can play it. A phone or laptop barely reproduces low sound, which I wrote about in Headphones vs Speakers for Ambient.
Try it: Ocean is the world built around the low end. Use headphones. It is free and starts in one click, with no account.
Open Ocean, free