You should always use "pnp=0" because that parameter is present when the generic PnP code is enabled in the kernel. If you have an older ALSA version, this parameter might be named acpipnp instead of pnp; see the output of "modinfo snd-mpu401".

There is no reason why the driver should refuse to load because it doesn't actually check for the presence of the hardware.  This seems to be a bug in Demudi/Debian similar to the snd-virmidi bug.