echo mia s/pdif
If you want to use the 2 channel s/pdif input on the echo mia and create a higher quality signal than the stock internal a/d converters on the mia's other 2 analog inputs, then an outboard a/d converter would surely be the answer. Dedicated units like the Lucid, Apogee, Benchmark, etc have been extensively reviewed on this board.
Another option to give you another preamp flavor would be to use a 2 channel mic preamp with built in a/d conversion like the dbx 386 or apogee mini-me.
A cheaper all-around option to increase the inputs to your computer would be to change out the mia card to something like an echo layla or rme multiface to increase your analog I/O to 8 channels. Both of these units can be used with a PCI card for your desktop pc or via a PCMCIA card for a laptop. I have used both and highly recommend the rme for sound and stability.