Really depends on what kind of setup you want to have.
I use a Phonic AM55 linked to a Yamaha UW500.
This is a great setup for me, because I can mix my MU2000's digital toslink output with a computer's analog output and send them both to the main analog output (going to my speakers), and convert the analog line to digital (USB to my modern PC for recording) all at the same time with no lag or dropouts. The UW500 also doubles as a USB-MIDI interface for my modern computer as well.
CM-64, CM-500, SC-55MkII, SC-88 Pro, SY22, TG100, MU2000EX, PLG100-SG, PLG150-DR, PLG150-AN, SG01k, NS5R, GZ-50M, SN-U110-07, SN-U110-10, Pocket Studio 5, DreamBlaster S2, X2, McFly, E-Wave, QWave, CrystalBlaster C2, Yucatan FX, BeepBlaster, SuperOctet!