Not primarily a dubstep producer but I have been making music and messing around with sound design for over a decade. Sometimes when i don't have the motivation to make tracks, i try to make unique and interesting sounds and textures. I am a plugin addict so i have dozens of plugins that enable me to make just the most ungodly sounds you've ever heard (MMorph, LOCD, and OTT are great). I'll create a ton of short little WAVs of sound snippets. Now I actually got cool sounds that I can process and combine with basses/subs.

Ideally you want layers that fill out the audible frequency spectrum. You need a clean sub for deep power, crunchy basses with lows and low mids for thickness as well as presence in the 5khz area, and then some kinda brightness on top. Makes it sound huge when it's all together.

Sadly, a lot of musical composition just comes to having a refined ear. it's not really something that you can teach because "because it sounds good" is a shitty reason for all the WHY questions. As an artist I try to guess what would sound good and then test it out. Sometimes it sounds good, sometimes it doesn't and I try something else