Randall RG80XP problems

I'm having a problem with my RG80XP. It was sounding fine yesturday, but recently, it started to have some problems with the tone. It sounds like the presence knob is dimed even with it set at 0. When I tried diming the presence knob,it pretty much was like adding another layer of presence on top the the already problematic layer.

Anyone had any problems with this and were able to fix this?