That's not where my search ended. As it turns out, digital in-/out-puts
are a bit of an issue, and since I'm replacing another device (with
neither MIDI nor S/PDIF---this is why), I want to be sure the new device
will have superior audio characteristics. I found a device that may be
exactly what you want: the Focusrite Scarlett 2i4 (see link below).
People say it has excellent sound quality, it works out-of-the-box with
Linux, and it's a USB 2.0 device with MIDI ports. To get the S/PDIF
outputs, I think I'm going to get the similar 6i6 model. The 6i6 is
apparently a bit of a struggle to set up, but somebody has to do the
testing, and it might as well be me.
As always, I'm curious to know about other people's experiences.