On my JBL 4333's I have replaced l-pads on horns and tweeters. One horn will have a lower output until I rotate the l-pad to full volume and then turn the output up. The horn will sound intermittent for a moment then return to regular volume. I have tried to take it out of the cabinet, but the square metal cover on the back won't give. I'm in Vancouver, BC and am not sure if I can do this repair myself or seek the aid of a professional? Any ideas?

Thanks

Peter