The 840C has a very slight bump in the treble, so it might be a little more clinical as compared to other warmer DACs. It's not sharp enough to give me any problems with the HD800 though.But if you're looking for something warm, lush and inviting, it may not be the best choice.
That being said, I just sent off my 840C for modding by a local enthusiast. He claimed that the mods will make it more analogue sounding as he's putting in an upgraded transformer, two EA Ultima clocks for the transport and DAC, upgraded capacitors/diodes/opamps and Furutech rhodium AC inlet and fuse. If you're interested I can do a brief review when I get it back.