Owning two Cracks w/Speedball I use two different DACs with each system. For my System I, I first run my digital audio into a DIP Combo (
http://www.monarchyaudio.com/DIP_Combo_Main_frame.htm) word clock and then into a Monarchy Audio NM-24 tube DAC
http://www.monarchyaudio.com/NM24_Main_Frame.htm. The NM-24 is a lot of bang for the buck and is very easy to mod (mine was modded by the Monarchy.) I actually got both and a couple of SM-70 MK II mono blocks, and some other gear for building the Monarchy site. CC, the owner, is now a one man operation and just does the work as a retirement hobby. Anyone in the Bay area should look him up. He is a very gracious human being, and has more gear/components in his huge warehouse than he knows what to do with. I am sure he could supply any DIYer with any component that he/she needed. I always make it a point to spend a day or two with him when I visit SFO.
For my other system, it will become my BH system once I test the Foreplay III and the two Paramount's, I use a highly modded E-Mu 0404. I swapped out most of the caps and put better OP amps in it. There are a couple of things I still want to do with it, but for the money (even when purchased new) it is a decent DAC with USB, TOSLink and SPDIF capabilities. There are a couple now on eBay for as low as $50.
I use Amarra Symphony on both systems. A Macmini (Lion) drives one, and a MBP (Lion) the other.