Decent DAC to accompany Burson Soloist SL Headphone Amp?

Recently picked up this headphone amp, which does not have a DAC.  I'd be interested in any suggestions for a DAC that I can connect to the Burson via RCA cables.  Smallish and good bang for buck (under $400) would be great.  This would be used mainly for computer listening - my CDP has a pretty good DAC.  Thanks.