The "official" dts cd done by DTS Ent. more than 20 yaers ago were (mostly)
1) taken from the original quad mix
2) and reworked in order to add a C and LFE channel: C as FL+FR and LFE as a lowpass filtering of all 4.
Usually these play best just killing C and LFE.
IIRC some Mystic Moods releases were true 5.1 new mixes.