I just bought a used Avatar 4x10 cab. I don't think the horn is working, but the price was certainly right. Maybe somebody here who's dealt with Avatar cabs can give me some advice here... The back panel has two 1/4" jacks and an unlabelled brass knob, which I assume to be an attenuator for the horn. Well, turning the knob seems to have no effect on the sound, so I'm assuming that either the horn, the attenuator, or the crossover might not be working. Can anyone give me some insight as to whether the knob is indeed a tweeter attenuator, and what steps I might take to diagnose and fix the problem? Thanks!