I have just replaced my small desk top speakers with an amp and floor speakers(Marantz PM6004 and Monitor Audio). Would a DAC make any further diference to this set up?
So are you using the 3.5mm analogue line output jack of your computer to the input of your amp, or some other type of connection?
If you are using the 3.5mm line out of the computer then a DAC probably will make an improvement - depends on the quality of your computer's soundcard really.
It is a external "audiophile" (in quotes) sound card and USB DAC, it has recording capabilty via its analogue or optical digital inputs, and also provides a separate volume control, headphone socket and microphone input. Here is a very detailed review http://www.hardocp.com/article/2011/09/20/creative_xfi_hd_usb_sound_card_review/1 ... if you check out the comparison graphs with a "standard PC" it is a worthwhile improvement.