Budget DAC with Optical Digital input

Putting together a portable system for some travelling I'll be doing this summer. I am getting a Sony Discman with an optical digital out, some Senn 600's, a headphone amp (undetermined, tubed would be nice). Something with at least a 20bit DAC chip would be nice, and the optical input is necessary. Detachable power cord is also necessary. Under $300 would be nice, under $200 would be perfect. If you have any suggestions or alternatives to my proposed setup please let me know, Eric