Some time ago I posted a question about my Ampeg SVT 410HE and the fact there was no variation in tone using the dial in back of the cab. Someone posted and said "check the lightbulb" inside. Sure enough, it was out and once replaced there is a huge difference. The horn really brings out the highs and you get a nice range with the control in back. Being a newbie and having bought the cab on EBAY without a manual I'm not sure what causes them to burn out. But they're easy to replace with a brake lamp from the auto parts store. Thought I would pass this along.