A DAC will offer much higher quality analogue input to the speakers than your current inbuilt Mac Pro Laptop analogue inputs.
I'd suggest the very good value for money and small "Devilsound DAC" for about $250 USD.
There's a review at: http://www.computeraudiophile.com/content/Devilsound-DAC-Product-Review
And also: http://www.positive-feedback.com/Issue41/ramblings_devilsound_virtue.htm
Among other places...
Just plugs into the USB port... presto!