I'm seeing some troubles with the filters on my Quad 33 pre amp. The fault is that when 5K or 7K is engaged, and then using slope the right channels cut's out almost immediately. It comes back when slope is set at 0. It works flawlessly on 10K. Is this probably a problem with the switches or something else? I've tried cleaning them, but from what I can tell it seems hard to get inside of them. Anyone has had this problem themselves or has an idea where to look next?