I am a huge fan of Onkyo, I own the TX-NR717, but the Onk's have had HUGE and well-known problems with the HDMI boards.
Before you buy an Onk I strongly suggest you Goggle the 828 and see if it has the HDMI problem (aka network problems, the network is affected by the HDMI daughterboard) I would tell you if I knew the answer, but I am not as familiar with that model as the older 717 and 818, which DO have that problem.
I'll let others who own the C2's recommend specific brand AVR's which are known to work good with them.
Glad I decided to go with the denon instead then ... Thanks for the input