I'm guessing here, as I'm not an electronics expert, that the surround circuitry in the HT receivers negatively impacts the recievers ability to play clean 2 channel stereo.After reading the link provided above, it appears others think the same thing.
I suppose it could also be that I just liked the sound of the Parasound processing equipment better than that of Denon. Maybe their 5.1 processors would do simple stereo just as well as their 2 channel processor does?