in your experience, have you found any loss in sound quality when watching dvds or blu ray when the player would downmix the multichannel audio tracks to output stereo from the player?
I can't speak to Blu Ray: I have a stack of discs, and no player (yet).
On the DVD side, I've had a number of discs authored to not allow downmixing. In every instance, it was the DTS track. So, I select the Dolby track instead, and all is well. As an aside, I'll take Stereo LPCM over anything else, every time.
In a sense, the answer to your question is "yes," because I'm having to settle for a Dolby track even though DTS available.
The Hollywood Sound Experience mixes vocals with a center channel very much in mind. Being in my mid-40s, I'm missing a few spot frequencies, and I don't hear dog whistles anymore. Otherwise, my hearing is in great shape. Every once in a while, I'll come across a movie with what is a truly bad vocal mix, no doubt hindered by my folding channels down.
I have recommended to a number of older friends who are slightly less militant about the room situation than I am that they go from a 2, to a 3-channel system by adding a center channel. Everyone has thoroughly enjoyed the move.