Cheap CPU headphones... can barely hear it, but the subs came through!
I would start with a saw wave. On/Off envelope (ADSR). Modulate the pitch with a retriggered ramp, basically pitch the static wave maybe an octave for the duration of the hit (so it's not flat). You can take a square wave with a very short release and layer that for a transient click. You can try to push off the attack of that square to emulate a softer hit, elongate the decay for a heavier hit, pull the release for a harsher, more abrupt kick.
Then you can start to mess with whatever synth parameters you have to customize it. You should probably go back to that simple envelope and readjust it to the tempo of your loop.