I bought a Phonic S710 speaker from Musician's Friend a while ago because it was on sale. I'm going to use it to run vocals through for my band, but we don't have a powered mixer to run it with yet. I decided to try it out by plugging it into the headphone output of my practice amp, which is only a 15W output. It didn't make any sound. My understanding is that the power amp in the combo should be able to power the passive speaker. Is it just not powerful enough, or not working through the headphone output, or is the speaker malfunctioning?

TL;DR version: My amp won't play through my passive speaker. Is the speaker broken or not?