there's no set path to every sound; but generally the sounds are made from base sounds we make out of synths, resampled, put into a sampler and played with (and yes - rinse and repeat) - we often just spend an afternoon making mids on dedicated sx projects, meaning you can use all your cpu on one sound, and bounce out a bunch of variations / sequences to be used as audio later or loaded into a sampler again. its all about getting to know your tools and slowly learning what you can do and how you can translate whats in your head into a sound, or just make accidents happen that might sound good ;)
and of course filters are important, but key here is:
the thing about basslines we like so much is the dynamics in the sound - movement. and that doesnt necessarily always have to come from filters, theres a million ways to make something change over time, for example just using volume changes run into a distortion plugin can work; ringmodulation, pitch modulation, anything goes really.
it wont help anyone if we start dishing out specific details on how a sound was made; it's all about finding your own taste in things, no one ever taught us anything about basslines... we just listened and fucked around.
and to round it up; learning to discern what you like and don't like about a sound (might sound obvious...but it's not!) and knowing what to do in the latter case is probably the most important thing to get into.
and the only way you can find energy to dedicate yourself to this long and exhausting process, is to have a lot of fun doing it

so you can keep it up longer without getting bored or frustrated trying to do something you think you should be doing.
(.......)
we eq out the low bass from kick drums gererally up to about 60-80 herz depending on the tune/sub etc, and the sub sits in the obvious regions... 30-70 herz depending on the notes being played.
distortion: we use our ears to get that evil distortion, there's not really a set path or plugin or whatever...
just to give an example, a few weeks ago nik and martijn recorded a reese that was actually an arp on a drum kit preset on the korg n5, so it was basically a bunch of drum sounds being played in succession rapidly and recorded into the pc way too loud. it sounded evil as fuck and somehow we could even tune it. you never know.....