I never was able to find a tremolo effect that was consistently usable on bass until I picked up this pedal for my guitar and tried it out with on my bass. This pedal truly sounds like the tremolo that you would expect from a vintage tube amp, due no doubt to the internal tube of course.
When you engage this pedal, the change is concurrently subtle yet well-defined. The warm, natural and rhythmic pulsations that this pedal produce defy logic, you would expect that sound this full and lush would have to be coming from a full sized amp, not a small little effect pedal!
The rate switch provides the choice of (wait for it) Slow and Fast rates which, combined with the Speed and Intensity knobs, really allows you to dial in the correct tremolo rate to match your song's tempo. The emphasis provides the choice of Normal and Emphasis. Normal provides a nice soft, warbling tremolo effect, while Emphasis churns out hard, choppy waves.
The speed and intensity knobs give a very broad range of possible combinations to bring just the right amount of warbly waves to your bass' voice and the tone control really helps you get your sound nicely into the mix.
I can't give a higher recommendation for a tremolo for the bass. I've tried more than I could accurately list but the Guyatone Flip Vintage was the only one that sounded natural, clear and kept the tone without sacrificing anything!
Guyatone Flip Vintage Tremolo