The first alternate setting reported by Audinst HUD-mx2 is an AC-3 setting (see lsusb -v output). It seems that pulseaudio always picks the first alternate setting by default, so we need to manually point it to the right PCM device.
/etc/pulse/profile-HUDmx2.conf
contents:
# http://mailman.alsa-project.org/pipermail/alsa-devel/2013-July/063676.html
[General]
auto-profiles = yes