Poop tons of compression, and sometimes probably just 2-tracked from a sp303 without proper mixing in some cases.
I think Madviilliany was mostly sp303>cassette tape.
The heavy compression was likely the vinyl sim effect on the sp303, and then some additional eq, and compression done at the mastering stage.
Pretty sure madlib cut the mixing engineers out of the process on many albums.
You can get similar sounds in the computer, but I'll admit emulating the mpc sound is far easier that the Boss/Roland sp line.
Just DL a ton of free lo-fi, tape emulation, and compressor plugins.
Nebula free, Ferric TDS, Wow&flutter, Time machine, Blockfish compressor, Boogex, Rough rider compressor, and density 3 are all a good start. I could go on. This is one area when DL'ing a ton of vst's isn't a bad idea. You literally need to just mess with all these plugs, and see what works for U.
Nebula is lush BTW, and seems to make all your other plugins sound better, but uses a lot of cpu. Put it on the master channel.