Get some nice sine wave sub sound with a lot of attack.
Knock the release and decay down so it becomes percussive like a low tom.
Make a 1 beat, repeating , 4 note random appreggio in a low register that you can feel as well as hear (make the second 2 notes in each 4 higher than the 1st 2, to emphasise the offbeat).
Add a teeny bit of verb, maybe some low fi to grunge it.
Roll off the top end.
Make sure it`s mono, with basslane or something.
Sidechain compress to the kick so it pumps just a little.
Hey presto, deep, rumbling hard techno bass.
Now just wack any old metallic sounds on top, crunch em a little, and there you have it.
Loop techno for beginners!!