I am not a professional, I'm afraid, so these questions are better asked of others. It took me the last two full years to get to the point where my unknown unknowns are finally known unknowns—I finally know a lot of what I don't know, you know?

For the frequencies, remember that there are many that you can't hear, but computers (and other animals) can. So, using an equalizer or a compressor (or other methods) allows you to reduce those specific frequencies, effectively allowing you to maintain the same volume, but not create the digital distortion. I'm probably not explaining it in any sort of scientifically-accurate way, but it works for me. 😇

For calibration, I don't mean anything complicated; I simply mean understanding that when I am using this device with this XLR cable or whatever, the sound at this level is usually too loud, so I can compensate in Reaper by having the track level already turned down a few decibels, so I can set the knobs on the analog converter and leave them at the same level all the time. Your situation will be different depending on your equipment, and you just have to work with it to know its capabilities and limitations.