IMO, the bitstream audio element of the best quality HD stream today is as good as the bitstream audio rate of DVD.
Blu-ray content is generally uncompressed. DTS HD Master and Dolby True HD (Meridian Lossless Packing) are lossless formats usually found on BD audio tracks. They will always sound better (with or without THX post processing) than any lossy stream no matter how good the streaming CODEC.
If your processor detects a lossless bitstream, it should be one of the above, otherwise it will fallback to a lossy format. A good processor will also down-mix if you don't have all of the recorded channels without dropping to a lossy CODEC.
Similarly, you can pass the conversion into analog external inputs without losing anything and do the THX post processing and room equalisation at the HT processor if your upstream source is better at doing lossless DAC. Personally, when running an analog signal in via external inputs, my preference is to keep it in that domain and do nothing more to it. Eg multi-ch SACD, BD-Audio.
