Well I think that "best desktop setup" depends on your taste when you are ready to spend thousands of dollars for the HD800. For example one of my friends likes the Burson Conductor 9018 very much because it is very detailed and smooth; while another one is wishing for a HDVD800 as a Christmas present. However if you want something extremely detailed and neutral in this price range, you should give the Benchmark DAC2 HGC or DAC2 D a try. Of course in the future you can add an amplifier to alter the sound if you want because many people find the DAC2 HGC too cold for them to enjoy music. I've been using my DAC2 HGC and balanced B22 amp to feed the HD800 for about 10 months and I think I won't need to upgrade anymore.
I own a pair of HD800s and the DAC2 HGC. I recently tried the HDVA600 with the DAC2 and, to be honest, if there was a difference in the sound of the HD800s by adding the amp, it was so small that it wasn't worth the $1400 for the amp, so I returned it. of course, YMMV.
I've since purchased a Headamp GS-X mk2 and there is a definite improvement in both the sound of the HD800s and in the sound of my desktop speakers (Focal CMS50) compared to being driven directly from the DAC2. I know the GS-X mk2 is supposed to be "wire with gain", but adding the amp/preamp to the mix seems to give my set up a warmer sound.
That said, the DAC2 HGC is a perfectly good DAC/preamp on its own and well worth the audition from Benchmark, especially given their 30 day guarantee.