Firewire Issues

My system configuration is as follows:
MACPRO with Cubase 7.5
Yamaha Motif XF8
Yamaha Tyros4
Steinberg MR816CSX
My Firewire software version is V1.7.3, My IEEE1394 software version is V1.08, MR816CSX is main version V1.10 and IEEE 1394 version V1.02.
The audio output from the Motif XF8 is connected to the MR816CSX inputs 5 and 6, The audio output from the Tyros4 is connected to the MR816CSX input 7 and 8. Audio from both units is heard through MR816CSX headphone output 1
When I start Cubase 7, the audio from the Tyros4 is no longer heard from my MR816CSX headphone output
Somehow Cubase 7 disables that audio path. What is the reason and how to resolve this?

Thank you.

Hmm, are you running this off 6-pin firewire? I don’t even know if the MR816 can run off firewire power, but if so, try connecting the power lead, your mac might not provide enough power over firewire.