Originally Posted by
89grand
A: You can't adjust the headunits output, so as long as the volume knob is not super sensitive, like way too loud with little volume level, or not enough output when turned all the way up (assuming the headunits output remains clean near maximum) then that's really all you need, so adjust the gains to achieve this. In other words, lets say a headunit goes from 0-35. I don't want the maximum system output with the volume at 20, but I also don't want to go all the way to 35 either. I'd like a volume level somewhere around 30 to be maximum for typical music, to leave some volume level for low output recordings. There really isn't any reason to want to be able to turn the volume all the way up, and still have no distortion, as you'd be leaving some output on the table. I like to be able to get a little clipping so I know the point when I'm getting maximum clean output.
B: Don't overdrive the DSP outputs by turning them to maximum. Somewhere in the middle is a decent starting point. From there, adjust the amps gains to level match the drivers for the most part. If the system sounds good, has no system noise, and has all the output you''d expect, you're pretty much good to go.
C: Once you start tuning, you will probably need to adjust some levels a bit, I'd use the DSP here for fine tuning output levels, or the amps gains if it is needed for some reason. What DSP are you using?
D: I'm not sure about the 400hz spike you're seeing. I doubt there really is a spike like that on the DSP inputs even with no source connected.
Turning the preamp gains up, so that you can turn the amps gains down really low is generally not of any value. Now if you have system hiss unless you turn the amps gains down, then trying turning the preamp (DSP) gains up until you lower the system noise. If you don't have system noise then don't worry about where they end up being.