no, i don't misunderstand.

i've never had render settings do anything negative to a part of my image.  i have had them improve parts.  that is, i have changed my settings to improve my results (and take more time), and sometimes portions have needed that improvement but others have not.  i've never, even seen a setting that would some how improve one portion of the image but not another, except maybe texture filtering.  and that's by material in 7 and up.

i've never seen anyone post render settings that gave problems that weren't computing power or a lack of quality.

so what settings are you finding make your scene "go to crap" but work for the figure?  could you show your results?  i'm very curious.