I just watched the Blu-ray disc Resident Evil: Apocalypse.  This is a Sony release with two audio tracks: English PCM 5.1 and English 5.1.  The former is of course uncompressed.  The latter is Dolby Digital 5.1, DVD-quality lossy compression.

When listening through the analog inputs on my receiver, I can use the pop-up menu to do an A/B comparison.  There is definitely a difference.  First of all, the PCM track is louder.  Also, it has a larger dynamic range.  This movie is loaded with sound effects, and they've done a nice job of taking advantage of all the 5.1 discrete channels available.  There really is no comparison.  The difference may be due to the lossy compression, the DD 5.1 decoder in the BD player, the DD 5.1 compression algorithm employed by the authors, or the audio engineering done by the authors.

There really is no way to tell which parts of this difference are due solely to the DD 5.1 compression itself. 

In other words, if the producers of the BD were so inclined, they could produce a BD that had the best DD 5.1 audio engineering possible, ditto for the PCM track.  In that case, the type of A/B comparison I'm doing here would then be as true as possible, assuming the BD player had a good DD 5.1 decoder.

I read an article, but now cannot remember where, describing a process where the author and his crew visited Dolby labs.  The facility had a state-of-the-art listening room where A/B comparisons could be done of their DVD-quality lossy format (DD 5.1) versus their new lossless format (TrueHD).  They then visited DTS labs and repeated their listening experience comparing their DVD-quality lossy format (dts) to their new lossless format (dts-HD Master Audio).

The author reported that under these ideal conditions, only the slightest difference could be heard between the lossy and the lossless formats.  Moreover, the author stated that, for the reasons discussed above, the home listener can't really perform a valid comparison.