Steve,
Please forgive a long-winded discussion regarding surround sound, but first let me issue a disclaimer: First and foremost my home theater is for movies, particularly action movies. I do not consider myself an audiophile, nor do I have a trained ear, but I do occasionally use the theater to listen to music, multichannel music. I never listen to two-channel music even if the source is two-channel I always upconvert it to 7.1 channels. From my experience there is a very good reason for this.
Two-channel music recordings require a fairly lively acoustic environment to sound its best. This is because for two-channel music ambiance, or the sense of spaciousness, is created by bouncing sounds off of the room boundaries. In contrast for multichannel recordings ambiance is actually deliberately recorded into the movie soundtracks, particularly the surround tracks. For multichannel recordings the practice is to ideally listen in a room with a moderate to a heavy amount of acoustic treatment to absorb reflections. This minimizes inteference with the direct sound on the recording. So my theater is a relatively "dead" acoustic environment. If I were to listen to two-channel music in my theater it would sound less appealing because it would sound unnaturally "dry". In my theater switching on surround processing to a two-channel recording restores a greater sense of spaciousness and always sounds better. Better yet 5.1 discrete music sources sound incrementally better than upconverting two-channel, but I found that depending on the quality of the mix upconverted two-channel can sound remarkably like a discrete multichannel recording. Your reference to the Beatles Love album is a great example.
On one embarrassing occasion a friend, who is a surround expert, suggested that I conduct an experiment to see how two-channel music sounds upconverted by Dolby. I told him that I only listened to discrete multichannel music in my theater because I felt it was clearly a superior experience. Anyway I went out and purchased the Beatles Love album because it was remastered in 5.1 and put it in my Universal Blu-ray player. The sound was fantastic as I expected. However, it turned out that the album came with a two-channel disc in addition to the multichannel disc, and guess what...I had inadvertently put in the two-channel disc. My system is setup to run Dolby if the source is less that 7.1, so what I was listening to was actually upconverted two-channel and the surround processing was so good I couldn't tell.
With this as background let me say that as much as I enjoy the Model S sound system I have not yet heard that improvement in spaciousness that I have experienced in my theater. However, I don't hear a loss in quality by switching on Dolby, what I hear is a reduction in the front soundstage volume as content is redirected to the surrounds. This lowering in front speaker volume may have led others to infer a loss of quality. Turning the volume up should restore the lost "quality". Right now I find that Dolby neither helps or hurts. I don't know the reason, perhaps it has to do with the small, and highly reflective acoustic environment of the cabin, or that the flavor of Dolby that Tesla has implemented is simply very basic, or of course both factors may be in play.
With regard to your remark that "5.1 discrete music sources (SACD/DVD-A) put much more emphasis on the rear channels than movies or "ambient" processing algorythms do.", I don't entirely agree. Yes, its true that surround processing such as Dolby ProLogic or Lexicon's Logic7 may extract content from the main channels and redirect them to the surrounds, but the degree in both discrete sources and upconverted two-channel really depends on the mix. The type of recording, music versus movies, doesn't generally govern how much sound is directed to the surrounds. It is true that in general movie directors like to focus the bulk of the sound in the front to support the on-screen action, but in my action movies my audience and I frequently find ourselves ducking as bullets zip by our ears from and to the rear of the room. So there's no general rule. Likewise in musical performances the bulk of content should generally be focused in the front soundstage, but of course there are notable exceptions.
It should be noted that recordings have two basic types of content. Ambient effects and direct sounds. Recording engineers have to consider the appropriateness of steering both types of sounds to the various channels. For ambient effects frequently all speakers are used at the same time. For direct sounds generally we expect them to follow the performers which are usually up front. One of my favorite recordings is Roy Orbison's Black & White Night, but I'm not crazy about the unconventional approach they took to the mix. You may be familar with the recording. We have Roy front and center with the support of a group of other top name performers. In particular they have a female chorus positioned behind Roy, but their voices are aggressively directed to the surrounds, behind the listeners. That's not where I would have placed them.
Returning to the issue of the Model S, if we could get a 5.1 signal in the system I would be a little concerned about sending direct sounds to the soundbar mounted on the rear hatch. It certainly should be okay for reproducing ambiance, but for Roy's mix discussed above, I'd be concerned that the direct voices of the chorus would sound better in the door speakers rather than forced to that soundbar. Maybe it would be something that could be mitigated with intelligent use of the fader control.
Larry
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