My two cents. DVD-A is a very flexible format. It can present lots of different sample rates [44.1, 48, 88.2, 86, 176.4 & 192 kHz...although the last two only in stereo] and word sizes 16,20,24 bits. The spec allows audio without encoding...in PCM for stereo [up to 192 kHz/24-bits] and surround [up to 48 kHz/24-bits]. Where MLP comes in [and yes it is a codec but a lossless one] is when you want 96 kHz/24-bits in 5.1 channels...it brings the total bandwidth down to within the max allowable on a DVD disc [around 10 Mbps].
Producers have the choice to use MLP or not. In fact, you can use MLP for a 48 kHz/24-bit PCM track just to make it smaller on the disc. MLP decoding is only available on DVD-Audio players.
96 kHz/24-bit PCM stereo is playable on DVD-Video players and does qualify as HD Audio [my preferred name these days], but only if the original recording was done at that spec or better. Taking an older analog tape or low resolution digital track and re-capturing it at 96 kHz/24-bits gives you everything that the original recording but is limited to the specs of the original. So, while I love older recordings that we well done...they are all limited to the analog tape spec or the low resolution digital spec.
That's why new tracks and new distribution methods [DVD-Audio and SACD] need to survive OR downloads at HD quality [iTrax.com].
My thoughts on the SuperDISC format. The Monster guys [both Noel and his son Kevin...I know them both] are trying a marketing approach to the world of HD Audio...but the tracks on their discs are limited to the audio specs of the DVD-VIdeo format [DTS, PCM at 96/24, DD] AND to the fidelity of the tracks during the original sessions. They do include the Dolby Headphone mixes, which is interesting but doesn't work for me. The Audience and Stage mix concept was "borrowed" from my label and that's a welcome move but the ultimate experience of HD Surround Music is not possible. Calling something HD Audio doesn't necessarily make it so.
I have all of their products [they gave them to me] and they've make a good attempt. But what the world needs is the REAL McCoy!