Maybe I'm doing something wrong but I haven't had to touch either the human or cpu sliders for coverage to make the defense on either side play better or worse. I still have good solid games. I blow out the teams I should blow out (more times than not) and have a tough time with those I should have a tough time with. The only sliders I adjusted were the fatigue and clipping sliders. Other than that, I'm still playing with the default all pro sliders.
I won't make adjustments until after I see how I do in a franchise year. If I end up blowing through the teams then I will make the adjustments. If not, they will stay where they are. Waiting on roster update to do that though. My db's are fine. The problem is, people try to control the db's all the time. Here is a bit of advice, let them do their thing. Look at their ratings and then match them up on the right receivers on the opposing team. Double receivers that need to be doubled. However, if u have good db's for the most part, let them do what they were designed to do, play "D". Db's don't make every play all the time. If they did, u would be playing an arcade game.