Problem with 5.1 sound

[the[V]oid]

Hi, I have a problem with my 5.1 setup. In mythtv's options I've set "alsa:default" as the device. This works great with 2-Channel-Audio, it's being upmixed to 5.1. But when I want to listen to a real 5.1 source, I only get 2-Channel-Audio, I think because the "alsa:default" device only can handle 2 channels. When I set the device option to "alsa:surround51", 2-Channel-Audio doesn't work. Is there a possibility to use "alsa:default" for 2-Channel-Audio but "alsa:surround51" for 5.1 sources?

Thanks!