Steinberg FW and USB aggregate device Mac

I recently got a CI2. I use a Yamaha Motif XS Rack with FW card for 4 outs(2 stereo pairs) when I play live. The Rack lacks inputs(physical inputs) so I was hoping to use an “aggregate device” in OS X(latest) on a 13" MBP, however in doing so, I get it working but there is instability(a kind of nasty metallic digital signal kicks in and out). I want a good quality Input(for vocoder etc…) So has anyone got this working or could Steinberg please check into to this as these are both Steinberg/Yamaha devices? Is there a Mac OS X tweak or config I could use. Cheers!