is this website right or wrong?

ill leave that one alone.

There is some truth to what he writes, but in general I disagree with him.  Gain controls are potentiometers, but they are not meant to be used as volume controls.  Matching the gain controls on the amp to the headunit as described in here will give you the highest signal to noise ratio, and will help prevent clipping.  Clipping is still possible, but set properly, the deck and amp will both clip at about the same point, so neither one is the "weak link".

His one example of a "crazy installer in the 80's" does not mean every other installer in the world is going to do the same thing.  It sounds to me that he's stretching to try and prove that his way is the best way- period.

Another issue is 3/4 of the way up on the volume knob is an arbitrary number that may be well before or well after the clipping point of the deck on any particular source (radio, cd, etc.).  Is the deck a factory, or is it aftermarket?  A factory ford radio, for example will usually distort at or before half way on the volume.  It's best to listen for the point at which distortion occurs, and back off a bit.

I vote on crack.

He is not incorrect when he says gain adjustment by ear is a good way to do it and that scopes etc. are not always required.  But he is WAY off when he calls gain a "volume control" and implies it has the same function, or when he suggests gain adjustment using a music source.  Maybe his real motivation is he wants to sell more speakers.