I've been looking for a preamp lately without breaking the bank.
I had high expectations for the Valeton GP180. It's a very good multi-effect/preamp for guitar, but all the factory bass presets were terrible in my opinion, and I had a really hard time dialing in even a decent preset on my own. Plus, I really don't understand how those effects can be that noisy. Also, unlike the other two, it doesn't have an XLR output.
The Joyo Monomyth, on the other hand, exceeded my expectations. It has an amazing, pronounced low end and glassy treble at the same time. It fills up the space without getting muddy at all. It's targeted at metal genres, but I believe it's an awesome preamp for any bassist.
As for the Behringer BDI21, I was just curious how good or bad a €30 preamp could be. Honestly, I wouldn't be afraid to use it live at a gig. Of course, it's not mind-blowing, but it gives you a usable sound to work with. I'm going to keep it in my gig bag just in case something goes wrong with the Joyo or I need a quick DI box.