I initially got this pedal hoping to make it part of a new guitar sound for a project that I'm working on. I hated it on guitar. I tried it on bass and hated it even more. So it sat for a month. One day, I picked it up and said, "OK, pedal, I'm going to plug you in and cycle through every bass & guitar and see if any of them can redeem you, otherwise, you're going to a farm upstate"
Cutting to the chase, with the right bass/ pick up configuration or blend, this pedal truly comes to life! I found that it really likes 2 passive pups where the blend is neck 100% and the bridge is between 50 and 75%. It also does very well when you solo the bridge pup.
No matter what settings you chose, however, it is important to understand that this is a dark sounding fuzz. Even with all of the pots at 100% (the pedal's "brightest" sound) it is still as dark as a raincloud.
When you roll the Variac control all the way down to 5V, you get a broken, chunky, almost burpy, synthy type of fuzz. With that pot set to its midpoint at ~10V, you get a much cleaner (but still dark!) fuzz and when you roll it all the way to 15V, you get the full effect of the fuzz and, yes, it is still very dark. Not muddy, not burpy not synthy... just as dark as pitch.
To be fair to the reader, while I have harped quite a bit on the darkness of the sound of this pedal, the Tone, output, and gain controls provide lots of sound-shaping options, so you aren't stuck in the muddy end of the pool! There are some great bright(ish) tones that you can get with this pedal. But if you are looking for subtle or transparent fuzz, this is not the pedal for you.
Overall, this is a very niche pedal to add to a bass pedalboard (or even a guitar!) but if you have the need and the ear and patience to dial it in, you will end up with a very unique sound that you can bring to the table to really bring a song to a new level, tonally!
Mxr M236 Super Badass Variac Fuzz
Fuzz Pedal for Guitar with Tone, Output, Gain, and Variac Controls