If it's just one speaker, do this.
Swap the inputs on the two speakers, ie plug the left channel into your right speaker and vice versa. If the same speaker gives you hiss, then it's the speaker. If the problem jumps to the other speaker, it's upstream....
Swap the output of the mixer, ie plug the cable connected to the left speaker into the right channel out of the mixer, etc. If the same speaker gives you hiss, then it's the cable, otherwise it's still upstream.
Swap the inputs to the Mixer, ie plug the left output of the Mbox to the right channel input of the Mixer (keep everything routed as before). If the same speaker gives you hiss, it's the board (likely considering it's a behringer). Otherwise, it's upstream.
Continue in this manner til you narrow down the problem. This type of logical deduction works wonders on tracking down all sorts of problems in the studio.