Hi
My first post here and hopefully useful!
I'm currently using a Focusrite Saffire LE which I find is great to use.
It's basically the same as the Saffire but slightly different i/o -
6 in/8 out. If I'm right, this covers your needs. It also has midi
in/out, but I haven't used that yet as I'm using an old Mackie interface
for midi over USB just now. It doesn't come with the onboard DSP (which
you can't use just now anyway), but because it doesn't have the onboard
DSP, it's slightly cheaper than the Saffire for basically the same
hardware. It also sound really good to my ears.
Focusrite also seem to be fairly pro Linux from what I read on the FFADO
site.