Ah you bring up some very pertinent and thought provoking questions. Indeed there are LOTS of paramaters and adjstments/tweeks to consider in a properly set up HT these days...and the difficulties keep growing as the system complexities do! These questions and their practical answers are the reasons that "leaving it up to the professional" is a very good, and even recommended option to consider!
Knowing "what does what", and the "why's", and "how much" and "for what reasons" is something you can only really nail down by having LOTS OF EXPERIENCE, time, and knowledge behind you. Ulitmately however, if one is interested in doing it themselves, as most end up doing (to only mediocre results on average sadly), you'll have to do your research, tinker lots on your own, and be prepared to make it a hobby! If that's acceptable and desireable for you, the rewards will end up being very much worth it!
Ultimately, the controls, settings, gains, set-up paramaters and such for a complex audio/visual system need to yeild PROPER BALANCE. BALANCE is the key, as with anything ultimately. A quick suggestion would be to set everything as close to a "relative-middle" as possible, and adjust from there if applicable. My experience suggests however that it's generally "promoted" by many experienced audio reviewers even, that you set the subwoofer volume control somewhere in the middle, and adjust from the processors level control for that application. Your results maybe different, depending on equipment used of course. For Video players and displays, I think the general trend is to let the player stay "flat", and adjust the video displays controls, which are likey more accuract, "fine tuneable", and involved probably. I've heard most professionals say they calibrate everything from the video display, and do miniman tweeking with the video source. Again, equipment might vary for best results.
As for proper level controls in general, with either video or audio, I find that if you set things properly from the beginning(most don't, nor will things ever be), you don't need to go "up and down" on the settings or controls! This is often a problem in most systems where the sub is up to loud, the system is too bright, the video display has too much contrast or brightness level, etc, etc! What you have there is a situation where there's limited dynamic range in the applicable display or playback system, simply because some portion of the audio/video spectrum is off balance, and tilted towards "high" all the time!...never allowing for delicate or low level playback. In the case of an audio system, where say the bass is too loud in proportion, you never get anything but constant loud booming, distorted, over-exagerated and dominent bass!...never balanced, articulate, even, and well integrated bass, that keeps the dynamic range in perspective and "wide-reaching"! The Same is to be said with video dispays where the contrast, say, is up too high, and everything ALWAYS LOOKS bright!..never suttle. There's no dynamic range here...and there's no middle ground.
Now as for gain matching levels and such, you don't want to loose dynamics with mismatched levels in an audio system. I can think of instances where getting enough gain to the amplifier is critical for proper dyanamic yield. This may however have more to do with impedence matching than anything else. Still, I have gotten better results with some equipment by, yes, adjusting things different than the norm...so experience and trial and error come in to play here.
You might want to consult old articles about such topics of interest, and recommendations are often expressed. Also, if you ABOVE ALL, consult your users manuals for your equipment, they often suggest the best mothod to use for best results with their gear!...that is a no loose recommendation right there.
Good luck
Knowing "what does what", and the "why's", and "how much" and "for what reasons" is something you can only really nail down by having LOTS OF EXPERIENCE, time, and knowledge behind you. Ulitmately however, if one is interested in doing it themselves, as most end up doing (to only mediocre results on average sadly), you'll have to do your research, tinker lots on your own, and be prepared to make it a hobby! If that's acceptable and desireable for you, the rewards will end up being very much worth it!
Ultimately, the controls, settings, gains, set-up paramaters and such for a complex audio/visual system need to yeild PROPER BALANCE. BALANCE is the key, as with anything ultimately. A quick suggestion would be to set everything as close to a "relative-middle" as possible, and adjust from there if applicable. My experience suggests however that it's generally "promoted" by many experienced audio reviewers even, that you set the subwoofer volume control somewhere in the middle, and adjust from the processors level control for that application. Your results maybe different, depending on equipment used of course. For Video players and displays, I think the general trend is to let the player stay "flat", and adjust the video displays controls, which are likey more accuract, "fine tuneable", and involved probably. I've heard most professionals say they calibrate everything from the video display, and do miniman tweeking with the video source. Again, equipment might vary for best results.
As for proper level controls in general, with either video or audio, I find that if you set things properly from the beginning(most don't, nor will things ever be), you don't need to go "up and down" on the settings or controls! This is often a problem in most systems where the sub is up to loud, the system is too bright, the video display has too much contrast or brightness level, etc, etc! What you have there is a situation where there's limited dynamic range in the applicable display or playback system, simply because some portion of the audio/video spectrum is off balance, and tilted towards "high" all the time!...never allowing for delicate or low level playback. In the case of an audio system, where say the bass is too loud in proportion, you never get anything but constant loud booming, distorted, over-exagerated and dominent bass!...never balanced, articulate, even, and well integrated bass, that keeps the dynamic range in perspective and "wide-reaching"! The Same is to be said with video dispays where the contrast, say, is up too high, and everything ALWAYS LOOKS bright!..never suttle. There's no dynamic range here...and there's no middle ground.
Now as for gain matching levels and such, you don't want to loose dynamics with mismatched levels in an audio system. I can think of instances where getting enough gain to the amplifier is critical for proper dyanamic yield. This may however have more to do with impedence matching than anything else. Still, I have gotten better results with some equipment by, yes, adjusting things different than the norm...so experience and trial and error come in to play here.
You might want to consult old articles about such topics of interest, and recommendations are often expressed. Also, if you ABOVE ALL, consult your users manuals for your equipment, they often suggest the best mothod to use for best results with their gear!...that is a no loose recommendation right there.
Good luck