Two channels on my 2014 Presonus 16.0.2 mixer have developed a buzz/white noise. It started with one channel about a year ago, and the same problem just started happening with another channel. The signal from a microphone plugged into one of these channels is much diminished, but slightly audible. When turned all the way down, the input meter still reads about 20% on each channel. Have the pre-amps on these channels gone bad? How can I fix this? Thanks!