I suggest eyeballing the levels and writing down the settings when you get it the way you want. Part of the confusion can also result from some disks with blacker than black content and others without. It is your choice whether to adjust things with the seeing of blacker than black content in mind.
In digital video, black can be mastered as 1 or as 16 on a scale of 0 to 255. (Maybe some non-standard values too.)
In analog video, black can be mastered as 0 or as 7.5 on a scale (IRE values) of 0 to 100. (Maybe some non-standard values too.)
Are you saying that these DVD's were made from analog masters where IRE 7.5 was matched to digital 16? If so then blacker than black (IRE 0 to 7.5) can fall between digital 1 and 15. If you adjusted digital 16 to be display black (using the TV menu), then you will of course bury any blacker than black content.
Or are you saying that IRE 0 was matched to digital 16 but black in the source was IRE 7.5 which would then translate to digital 32 or so? Here source black would appear as dark gray but then blacker than black would be in the 16 to 32 region and should be readily reproducible.
Within the range limits, the brightness and contrast controls let you spread the source black to white range over the display black to white range, for example if incoming black happened to be digital 32, then you can put display black up at 32 if you wished to. (You would do this visually since the numbers 7.5, 32, etc. are probably not shown on the screen or in the menus.) I would not be surprised if there are some non-standard masterings resulting in several different settings to write down and use for different disks.
Remember when lines and logos burned the TV screen? I was at a concert where a musical selection made extremely heavy use of about four of the keys of the piano.