The content: I have been studying this for several years.  Many of my reference recordings are in the 'commercial pop' category (Russ Freeman / Rippingtons, 'Curves Ahead', Weekend in Monaco'; Anita Baker, 'Rapture'; Andy Narell, 'De Long Time Band', and others you know about better than I), and these have the regular commercial pop mix; 3dB peaks at ~70Hz and 5KHz. There are lots of otherwise fab (see above) recordings that our rigs must reproduce without prejudice. Then there are the Mercury L P, Deutch G. , and so on (Jackie DuPre Elgar Concerto, Barbirolli Sebelius 2, John McLaughlin Bill Evans tribute, Lauridsen 'Lux Ǽterna', and others you know about better than I)