As far as I understand (understood) it, a DTS CD does indeed contain a 20 bit stream when encoded and it has nothing to do with the type of master it came from. The CD medium has a maximum storage capacity of about 800MB. The standard bit depth is for an audio CD is 16 bit with a 44.1 kHz sample rate, and this will provide about 80 minutes of music. The DTS system encodes a 20 bit data stream, but compresses it in a lossy fashion, so much so that it can fit a 6 channel 20 bit stream into the space afforded by the CD medium. The compression process is supposedly psychoacoustically selective with the data it removes. It then wraps the compressed data stream in a 16 bit container to fool the player into thinking its just a normal CD. When the data stream is played back, the DTS capable processor recognizes it as a DTS stream and decodes it accordingly. If you play it on a standard CD player you just get noise because the player thinks its a normal 16 bit audio stream, but it isn't.
The reason to upgrade to MLP or a higher resolution is to provide for both a higher sample rate (to 48kHz, 96kHz, and higher) and a lossless compression scheme, in addition to increasing the bit depth from 20 to 24. But all this takes up a lot more space than what is available on a CD. So it has to be written to a DVD, which has a storage capacity nearing 5GB, several times the space available on a CD.
There ARE indeed CD's sourced from 20 bit masters, but they are encoded as standard 16/44.1 and will play on any CD player and have nothing to do with DTS. The 20 bit data is simply down sampled to fit in the available space.
And if i'm understanding your equipment performance comments correctly, the difference in sound quality from DTS 24/96 and MLP 24/96 is greater on your Meridian player than it is on your Oppo, yes? How do you know the Meridian player simply isn't better at playing MLP than it is at playing DTS? Or for that matter, how do you know the Oppo isn't better at playing DTS than the Meridian? You would have to A/B the Meridian DTS stream vs the Oppo DTS stream.