The stock DV on your 9000 fits between the intake pipe and the pre-compressor pipe and is rec-circulating. Indeed the stock configuration is the same on a C900.
When you fit a one port (atmos) DV to the intake pipe, you need to block up the (would be) hole in the pre-compressor pipe.
The blanking plug on the C900 for this is buried at the front of the engine right next to the intake for the turbo compressor, and is difficult to see.