Ahhh, I see... DTS HD... My memory had "lossless" instead of that.
Theoretically, it could be lossless already in this format, but it's impossible to tell without full details of the file(s).

http://en.wikipedia.org/wiki/DTS_%28sound_system%29#DTS-HD_High_Resolution_Audio

For those of us with a PS3 as blu ray player, this is also some good reading:
http://www.engadget.com/2009/04/21/hd-101-how-to-use-dolby-truehd-and-dts-hd-with-your-ps3/

Either way, I think that's a plenty good format! Not really sure there's any benefit to messing with "better" than this, esp. keeping it at ~25GB. Any more, and it might be too much to fit nicely on a single layer disc. ...perhaps it'd be best saved for version 2.5/3.0/??

TBH, I don't even have a receiver to do this stuff, but it's "nice to know"

There's an old saying. "If I were a betting man" ....I would bet that this DTS HD file is already lossless.

As for lossless mono, I'm personally a little "meh" on it. Cool stuff that there can be so many good choices on one shiny little disc :)