The Dialogue Normalisation option of AC3 encoding can be used to reduce the playback volume. The ideal way to determine how much reduction is needed would be to measure the RMS level of the audio files. For example, a while back I compared the '93 PCM to the mono mix, and found the '93 (and consequently also the 70mm) to have an RMS value of -25.1 db, while the mono measured at -20.5 db. This means that the mono file will sound 4.6 db louder with the receiver set at the same volume level. Using the Dialnorm function to reduce the mono version by 4 db or so will compensate for the discrepancy, making them sound closer during quiet sections. This will also cause the superior dynamics of the 70mm to be much more apparent.
The setting for no reduction is -31, so a 4 db reduction would be obtained by choosing a Dialnorm of -27. I imagine a similar setting would probably work for the '85 version also. Make sure, however, that dynamic range compression is not used for any of the encodes.
The Dialnorm function is something that can cause confusion among casual listeners, because the inclination of the ear is often to associate "louder" with being "better". The AC3 audio on the GOUT is lowered by 4 db compared to the laserdisc PCM, and it is easy to mistake this difference for a reduction in dynamic range.