If I understand you correctly, the 7.1 analog inputs on various receivers should do what you want, even though you are not using all the inputs. L/R and sub analog signals should be passed through without being digitized, at least that is the way these inputs used to operate. Since they are not used as much these days, the manuals often do not discuss whether or not they are digitized. I would try to find out more about the 7.1 inputs on the receivers you are interested in. I know the upper end Marantz have 7.1 inputs.
AVR :2-channel with sub full range: Source Direct
I am looking to play full range stereo recordings with sub in Source direct or analogue bypass mode...in other words...i dont want the signal to be "digitized" through DSP and want to bypass tone controls,etc...does Denon or Marantz past or present models currently operate like this?