ITunes For Mac :: Sound Quality And Volume So Different When Use Other Media Players?

I have spent a lot of time over the last two years trying to get my itunes library organized as well as getting good digitized versions from my vinyl records as well as CDs. I was pleased with my results and then I added the songs to itunes. I could not believe how inconsistent the sound quality was. Invariably the volume is always lower. [Do not post a reply such as: Check the volume controls in itunes proper as well as in itunes preferences (i.e., Sound Check, Sound Enhancer) & the Volume Adjustment in the Information pane under Options for particular songs.] I tried all of those means of adjusting volume. They do NOT work. 

There is something more insidious at work in itunes which prevent good sound reproduction. I will give an example. I have played Frankie Valle and the Four Season's song "C'Mon Marianne" on the OS X system sound (i.e. clicking the play button to the song file when selected on the desktop or wherever it resides) as well as in Amadeus Pro. The song sounds great. When I play it when it's in the itunes' library the first part of the song is too lound and then when Frankie Valle starts singing his voice is at a significantly reduced volume. 

I have edited the foregoing song in Amadeus Pro to a very high volume (i.e., Effects>Amplify), reloaded it into itunes and it still does not inprove the volume of Frankie's voice in the song, even though the rest of the song is somewhat louder. I have played with the Equalizer in itunes to no effect. It just does not work. 

Now, I would glading move away from itunes and use another media player. However, I have other Apple devices through which I listen to music. And the only way I know of to listen,e.g., on the iphone is through itunes. 

The other related problem with itune, or should I say iCloud, is icloud match. The icloud Match has taken great sounding music and given me back music of an inferior quality, even though it is a Matched AAC format. 

Info:
iPhone 5, iOS 6.0.1