I use one of these (actually looking for 1 more) as an MD in my Family Room. You are correct, it s the default behavior of the MD to use the internal speaker set, this is a limitation of the business class hardware that is the Evo.
What I have to do is use CTRL-ALT-F2 and get to a terminal on the MD. Login as root, run
and mute the Headphone setting. After I do that the sound switches instantly to the external source, and then to make sure it sticks, I run
Once this is done it uses the correct output on concurrent reboots.
Hope that helps. And if you want to get rid of one or two of those Evo's, let me know