InVision's method can be closely approximated by a person with a few year's experience.
The idea is twofold:
*to get all the input levels to be "the same," whatever that might be, so that you don't have to adjust the main volume when switching from one input to another
*to keep all adjustments within a range such that they don't distort. (This falls under the misnomer "gain structure" in PA and commercial usage, where you may have several stages, each with its own volume, and you need to be sure all volumes are up loud enough to conceal their noise floor but never so loud as to cause themselves or any following stages to distort.) (That's a misnomer because we never adjust the gain of any amplifier. Instead, we adjust the level of the signal going from one non-adjustable gain stage to another. But that's what we call it.)
If you have a component with an AM/FM tuner in it, that's your reference, since you probably can't adjust it, or, as I've been disappointed to find on a Yamaha Aventage receiver, even at its lowest setting it's louder than any other input trimmed all the way up.
Anyway, If you can get the CD input close to the tuner input, adjust a BD or DVD player to play at the same level with the same CD in it. Verify with a BD or DVD that this still works. Note that the system in surround will probably sound louder than the levels in stereo, so this entire exercise might be fruitless anyway.
Go to your different streaming sources and adjust them as well as you can.
Along the way, decide NEVER to use your phone as a source, especially for music downloaded from the internet, as that programming varies wildly in its level.
edit: notice what can happen along the way. It took some bravery to admit this: [Link: remotecentral.com]