I have an AC568 Twin that has been blackfaced. Is it possible to convert the Bright Switch into a Bright Knob to control the amount of brightness? If so how would it be done? Also: the reverb on my Twin has developed a low level buzz/hiss (120hz?) that increases with the reverb knob but not the volume knob. I've switched the 12AT7's, V4 12AX7, and send and return cables with known good, and cleaned the inputs on the amp, tank, and footswitch. Still there. What else could be the culprit? Reverb tranny? Tank? The reverb still sounds great except for the little buzz, which is noticable with the volume on 2-2 1/2 and the reverb on 4 or so.