Are you having this problem on stage or just in your bedroom? Usually playing live, the hustle and bustle of the audience and the other instruments in the band will mask quiet hiss and it's no problem.
How do you know the hiss is coming from the bass and not the Boss multieffects unit? Try plugging your bass into a high quality preamp (instead of the Boss) and see if that solves the problem.
Another approach you can take is to turn down the "treble" control, that should help.
Worst case, you can purchase a "noise gate" (if your Boss doesn't already have this feature).