this video is similar to the way you talk about but better, you just connect this piezo in line with everything, its a really high impedance so it doesnt affect the load but you can definitely pick up distortion with it. Its better because your sub will present the same load on the amp as it would be by itself. When you use the mid, its a different load on the amp even if the impedance is stated the same due to many factors not taken into account. However you need to analyze your music and see how strong the bass in the recording is. Generally rock and pop etc is around -10ish db, modern bass heavy rap is -5db to -3db, you use a free program called audacity and use its spectrum analyzer to see the strength in the musical recording which has a direct impact on your amp's power output and how you should set your gains. People will never get max clean power all the times because each song is different so you'll be fiddling with the controls for each song... Most people just give up a little max output for a general safe level all around which is why stuff like Oscope and DD-1 are popular, they are supposedly idiot proof but for people that dont understand why there's a -10 db test tone, a -5 db test tone and a 0 db test tone on the disk, its gonna spell trouble for the install.