At the end of the day, true sound is a bit like beauty. It is indeed in the (eye) ear of the beholder. We can rant and rave about what is correct and what is 'fidelity' but there are so many other personal factors that come into this. This however does not change the word fidelity!
Correct reproduction (fidelity) should be 'played as recorded', and with all the effects etc as it was recorded. However, this often is not the way we like to listen to music/movies. Let me give an extreme example. How many of us have heard a car go past with the sub-base pumped up 12 to 20db above the rest of the music? Some like to listen to sounds this way? I don't and I find it hard to fathom why others do, but this is just my opinion - (beauty in the ear of the beholder again).
THX gives us some 'measure' of correctness with regard to the replay of music/sounds recorded (with surround in mind) as the director or author of the sound intended. It gives us the belief that what we listen to is somewhat correct as the movie/recording should be, the impact is as it was designed and in keeping with the intent of the overall production (cd, dvd or otherwise).
I find much of the impact of a movie lies in the sound orchestration. The theme, acting and plot etc are paramount, but the sound adds to the movie and often compensates for heaps of other shortfalls . I hate to say this but sometimes the sound is the only reason for the movie (in particular some music videos - whose visuals are crap but the sound is great).
Many surround systems allow you to colour your surround system by way of enhancements etc. (mine does). This is a good thing, and it is also a bad thing. It is good in that you 'over you library' can set up your system so that the settings are as you like or that you can 'over your library' set them up so that they all sound all the same. If music/sound was supposed to be a 'one thing' the we could all go down to the shop and buy a box of 'one things' and be happy, all the music/movies would sound the same and the sales of 'one things' would soar.
Life is full of wonders and variety, thus so is music/sound and the ideas of movie directors with their sonic depictions of emotion, fright, impact, serenity, grace, distrubance.... need I go on.
I am all for a great surround system. I have one of the best myself. Surround is an enhancement over stereo and let us use it to gain a greater mood impact to the visual that we are presented with, but do it with forethought. Me, personally, I use my system on 'auto detect' and let it sort it self out according to the DVD or CD I insert (thus it becomes a decision of the director/sound editor not mine). If DTS, then DTS it is or PrologixII or whatever. If all else fails, I manually select Neo6
to get 'accross the board' and an acceptable sound that has reasonable fidelity and a good surround enhancement to make the movie/cd interesting.
...but these are just my thoughts...