Are you perhaps trying to have both PulseAudio use device A as its sink while also using
an audio application sending its audio output directly to device A via ALSA? If so, that
would definitely explain the conflict. When I recreate that scenario, VLC for example, gives
the following error.
With PulseAudio currently selecting the D10 as its output and playing audio.
vlc --aout=alsa --alsa-audio-device="front:CARD=D10,DEV=0" some_song.flac
produces
alsa audio output error: cannot open ALSA device "front:CARD=D10,DEV=0": Device or resource busy
Directing the PulseAudio output to an audio sink other than the D10 resolves the
conflict and VLC (used in this example) is able again to play to the D10.
It's quite possible that I'm failing to understand your circumstance, my appologies.