I just bought a used Mesa/Boogie 4x10 and I honestly can't tell if the horn is working or not. This is the first time I've had a cabinet with an adjustable horn, so I'm not really sure how apparent it should be when it's working. But when I crank it all the way up and stick my head right in front of it, I don't think I can hear it. It's hard to tell though because the speakers are so bright anyway. Is there an easy way to test it? If it's not working, what could be some potential causes, aside from the obvious one that it's not plugged in?