Hi s.k.,
So far as I can see, the output settings are configured according to your output device. If you change them by editing the mpd.conf file, the changed settings will only last until next reboot, when the file will be overwritten with what is in the MPD settings page.
Try moving the # to the line above and restart mpd. It will output 24 bits at the source's sample rate until you reboot or make any other changes to settings.
From what rern has said, it looks as if the resample settings will soon be added to the settings page, which will solve the problem. I have several DAC chips which I like to play around with.