so is there a difference between the DTS-MA and the Dolby TrueHD ?...
There is a thing called 'Dialnorm' short for dialog normalization. It was originally used for tv broadcasts to try to stop the commercials from being massively louder than the show you were watching, but for some unknown reason it was adopted and uselessly used in movie and music production. Dolby TrueHD much,
much more often has this applied for whatever reason, but it is still possible, although rare, that DTS-HD MA can have it too.
Dialnorm actually doesn't make the center channel any louder or quieter compared to all of the other channels. It just checks the overall level of the center channel and then raises
all channels equally in order for the center channel to achieve this 'desired' level.
The most common setting that I've seen is a dialnorm level of -4dB, which means there are flags in the audio telling your equipment to increase every channel by +4dB, so therefore if you have a DTS-HD MA track without any dialnorm applied, compared to a TrueHD track with a dialnorm of -4dB, then the TrueHD track will be played back 4dB hotter than the DTS-HD MA track. It's like it's saying that the center channel audio is 4dB too quiet overall, so it compensates for that by adding 4dB to all channels.
Audiomuxer just recently added an automatic dialnorm stripper if you process any audio files with that program. If you use that program to rip say a 5.1 DTS-HD MA track with no dialnorm applied to say Wav, or Flac files, and then do the same thing with a 5.1 TrueHD track from the same recording with the dialnorm present, then after the stripping, they should both sound completely identical.
Although it shouldn't really matter, I always choose the one without it, usually the DTS-MA or the quieter one, when ripping something to Flacs if there are multiple options. If there is an LPCM option, I always use that one, as DTS-MA and TrueHD are just losslessly compressed containers for the original LPCM audio.
P.S. - I realise this is off topic, but as this is not a poll thread, I thought it would be okay.
Edit: Here's where I found out about this years ago and explained much better:
http://forum.blu-ray.com/showthread.php?t=121087