Most games have their sound in LPCM, which is both unencoded and uncompressed throughout the entire chain ... so the reading you're getting on your amp is exactly correct, and is exactly what you should want to see. In this case, neither your amp nor your card is doing the decoding, because the signal was never ENcoded
There's a few older games I think that actually are encoded (and compressed) using Dolby Digital, and when playing those I suspect you'd get the Dolby Digital readout on your amp, but otherwise ... it's going to be 5.1 LCPM aka Multi-In on Dolby receivers. And that is better than regular DD or DTS.
As you suspected, there are no games (that I know of) that are encoded using Dolby HD or DTS Master so for the time being you should never expect to get the indicator for those when gaming. Those are Blu-Ray specific 7.1 channel encoding formats, and I don't think there's even any 7.1 channel games, in any sound format. And if there ever is, you're most likely going to see a 7.1 channel Multi-In reading on your amp.
And again with this scenario, neither of those proprietary encoding formats is inherently any better than 7.1 LPCM.Edited by brettjv - 10/29/11 at 12:46pm