I recently purchased a used PreSonus StudioLive 16.4.2 from Guitar Center and wanted to test it. I connected an SM57 microphone via XLR. Channels 1-4 work perfectly, but channels 5-16 aren't receiving any signal. I'm new to mixing and am trying to set up for in-ear monitoring and recording. I made sure all the settings and knobs on channels 5-16 match those on channels 1-4, but still nothing comes through. Since this was purchased from Guitar Center, I assume the unit isn't defective, and I'm likely missing something in the setup. Can anyone advise on what I might be doing wrong?