Exactly the same issue! It looks like "Medium Adaptive Volume" became "Night Mode".
I tested this for a bit of self-education on my HTPC by creating a new Zone with Dolby Digital turned off in Output Format, and used Mix Channels to downmix. Sure enough, I reproduced the error. Bummer.
So how about another solution PGibby?
Use your venerable 2 channel DAC for high-quality music listening, in its own Audio Zone. Then use a new Video Zone with output directed at something that can handle 5.1 (or 7.1) channels, such as the built in sound card on your PC motherboard. Then set up Zoneswitch to switch to the appropriate Zone based on the content being played. You could then boost the Center volume on Videos to better hear dialogue in movies, while still enjoying the best sound quality when listening to stereo audio sources.
Would that be possible? Is your DAC connected to an amplifier that could handle 6 channel analogue input, and does your PC have 6 channel analogue outputs, for example?
I know my motherboard sound card has both Toslink and Analogue output capabilities, although I'm not sure if I can have both connected at once and switch between them. The audio chip and driver may not allow that. But if it did I could output all stereo Audio (music) via the Toslink in 2 channel PCM, and all 5.1 channel Video via the analogue outputs, into the analogue inputs on my 1990's Receiver.
Perhaps worth thinking about.