I mean liba52dec downmixing.
That will do it, definitely. It is very rare to find an AC3 decoder that can be made to disregard the DRC profile when downmixing to anything less than the left, right, center, left surround, right surround layout. In fact, I would be surprised if such a thing existed at all outside of pro equipment. I spent a lot of time scouring through avsforum to find similar reports, and the explanation (and Dolby's own documentation) points solidly to the notion of forced dynamic flattening on the downmix. Ostensibly to avoid overloading the digital to analog converters, but I really can't see how that would happen if 24-bit DACs are used, unless the DAC counts its volume scale down from maximum and not up from the noise floor, so that even 16-bit dynamics could be pushed into clipping--but I don't know enough about that to comment in an informed way. Even so, I don't think I've ever experienced any problems with clipping when downmixing high volume DTS tracks into stereo, so it really seems like Dolby should have allowed for this 'helpful feature' to be turned off. Oh well.
As for Dialogue Normalisation, this is not defeatable under any circumstances, and is often a source of confusion about the dynamic range of a soundtrack. It is much easier to tell when, say, a loud LFE bass sound or other high volume peak is reduced than it is hear that spoken dialogue and other relatively quiet sounds average at a lower level--the ear becomes accustomed to it, and the perception of 'loud' or 'soft' varies accordingly. Witness numerous reports of the dynamic superiority of the 1993 laserdisc mixes compared to the GOUT: while it is true that the GOUT version of Empire is inexplicably flattened (Belbecus commented on this when he first made the PCM available, and I can confirm it), the other two films are not--the only difference is a 4 db gain reduction most likely attributable to Dialnorm. Adjust the playback volume to compensate, and I'll wager it would be much more difficult to tell the difference between the level-matched AC3 audio vs. uncompressed.
These factors are part of why I haven't wanted to make the 5.1 version generally available yet, because I haven't been able to test it and see exactly why the volume is reduced. Satanika assures me that no DRC or Dialnorm was used in the AC3 encoding, so I'll have to see what I can find out.
What were the main changes you re-created with this mix?
I've given a fairly thorough accounting of the work involved throughout this thread, but in short, the main source was the 1993 laserdisc mix, from the GOUT-synched PCM provided by Belbecus, which is essentially a 2-channel downmix of the 70mm soundtrack with additional sound effects added in. I removed these extraneous sounds (some of which are harmless while others prove distractingly unnecessary) using the 35mm stereo mix, also provided by Belbecus. Since the 70mm featured surround sound, and undoubtedly contained more bass than the 1993 version could accomodate, I put together a custom LFE track for the 5.1 version, the majority of which was sourced from the 1997 special edition, with some portions also derived from the 2004 dvd if I felt they worked better, and on rare occasions even isolated and amplified from the 1993 mix itself, for places where neither special edition proved satisfactory. The volume levels of these added bass effects were individually adjusted to provide an ideal blend with the older soundtrack, and to create as enjoyable an auditory experience as possible. Satanika took care of the upmixing from stereo to surround, since I can't get that to work on my computer.
See here for the list of changes made to the 1993 mix: http://originaltrilogy.com/forum/topic.cfm/70mm-mix-to-93-mix-comparison/topic/6501/