If I had to guess, foobar2000 is likely converting the 5.1 to a 2.0, which always has to go through a PCM stage (so DSD > PCM > DSD) and there's really no way to avoid it. If doing such conversions in MC, it also has to go through a PCM stage in the middle. DSD to PCM and PCM to DSD are lossy conversions.
One way around this is to use DSP Studio > Output Format and set it up accordingly. But do note, like mentioned above, if you convert the 5.1 to a 2.0 it does go through a PCM stage and you can't do it while bitstreaming. You can however probably setup a zone for this then use the ZoneSwitch feature to switch to it when trying to play a 5.1 channel DSD file. The other way around it is to find a 2.0 version of the track(s) if possible.