The audio research DSPre ha a built in DAC. Personally I would never buy one. I don't want to have to change my preamp if I want to upgrade my DAC.
The preamp is the control center of your system. A volume control on a DAC just can't compare to a truly great preamp.
I would suggest you borrow a Ref 5se from your dealer and listen to it in your system. Try it both ways and see which way you prefer. That is the only way that you will know for sure.