The only thing I know of is the logitech z series speakers with the built in DD5.1 and DTS decoder. Though of course that kills your choice of speakers. And since you are worried about the onboard DAC ...
The newest Dolby Digital (AC-3) multichannel decoder, the DP564, features Prologic II, the latest in matrix surround decoding technology. The new decoder is twice the size of its predecessor, the ...