Disklavier

I am getting a DKC-850 soon and was reviewing the manual (online - which by the way, can be found in the yamaha manuals section).  I'm going to be replacing my control unit on my DC3A.  There is a piano control plug that you connect on the DKC-850, but I also noticed that you have to connect the audio out of the DKC-850 to the AUX IN of the DC3A (my piano has built in speakers).  I currently use the AUX IN for other things.  Doesn't the DKC-850 drive the audio directly using the piano control.  Isn't that the way my current control box DKC55RCD does it?  It certainly doesn't use the AUX IN of my piano.  I can work around this, but this seems to be a missing feature?  Thanks for any feedback.  SJ