I ran a Stax energiser off an 818v3 while also feeding a pair of 8KSE's and using headphone mode worked perfectly, note that headphone mode actually brought the DSP's out of standby but at 0 volume.
Looking at the 218 manual (I do not have a 218) it does not appear to have a headphone mode, also in the manual is the following table


Analogue Out to amp or active speakers
SpeakerLink out to DSP's

and

Digital coax output, use this to connect to the digital input of another 218 or a Distributor 1, using a 75Ω digital cable.

So it looks like what you are trying is not documented as being available, sorry I can't be more specific.