Imo, you're assuming wrong. There should always be more air being removed than the one being put in.
So, in your case, you're putting more air into the case, wich has no sufficient exaust, then it heats up, and alot of the hot air is trapped inside your case.
Then, theres another problem, it's a cube. The air movement through cube server type cases are not that "regular". Some pictures would be nice

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Last but not least, the Coolermaster HAC-V81 is not that good as a HSF. There's alot better ones, cheap. Like a v7+ or a v9.