sub-bass

Hello, I am an aspiring amp/fx designer. I'm building my first amp right now for which I am going to be looking for in person feedback to help me fine tune the voicing, controls and features. I've already run a preliminary survey not too long ago here: The essence of my question here is: what...

I've always just played straight into a tube Ampeg so I don't know much about effects pedals for bass. But is there a pedal that can put a heavy low end effect on lower notes, that I think would be called sub-bass or a lower octave doubling?, and that could leave higher notes above a chosen note...