I used a guitar multi-effect processor, a Digitech RP250. I thought it wasn't going to work with the double bass, since it has so many low frequencies, but it did... Not with all the effects, off course, but these ones did. I connected it with a fretted strings pickup (it was originally designed for a violin, but I'd been using it on my viola with good results, and the double bass player is also a luthier, so he came out with a solution for adapting it to his instrument). I combined the parameters for my effects there, editing the presets...
Yes! That is precisely one of the moments in which I went away from the concrete ideas I had and the notion of development went to hell hehehe. It is exactly one of my weaknesses in my technique, and I'm working on it right now (just like when, on the instrument, your teacher picks a specific study for you because you need to solve a certain technical problem).