polini does not come with spring, but they have two available. malossi comes with yellow spring, and has others available.
i had tons of belt slip issues with malossi variator + malossi belt + yellow spring.
switched to polini variator + malossi belt + yellow spring and all is well.
i also bought the red spring and am thinking of building a clutch using it so that i can run heavier weights and tune this shit better.. but right now i'm in the if-it-ain't-broke-don't-fix-it boat.