I followed all the steps and have set my gains using a $7k o-scope I borrowed. Stock headunit, mtx req, then my re audio vlx-1de amp. I used a 50hz 0db sine wave I downloaded from somewhere (i forgot where). anyways when the test tone is on the bass is pretty decent (2 12" alpine type R's sealed, temp install until ported fiberglass is done). rap is good too but as soon as I put in a hard rock I get no bass even with bass boost turned all the way up which easily clips the 50hz tone.
I have been messing with audio systems since I was 15 which was 16 years ago and I never once used anything but my ears to set gains and I have never blown a sub to date. I have used good judgement on bass and distortion.
I guess this inst a question as much as it is just my experience using the o-scope method. I can see the benefit for someone doing an all out spl burp with a known signal and want to make sure there amp doesn't clip and blow a sub. But since music dynamic and is recorded at different levels, most of which aren't as high as 0db to begin with I don't see the point in using a scope for daily driver type systems.