I don't have CM 690 but I have an Antec case that has similar design of PSU position.
Antec's not smart enough to put an intake vent underneath the PSU.
I installed the PSU upside down with fan sucking air from the mobo. I think that is the usual thing to do. You don't want to waste your PSU exhaust power just to suck air and dust from the floor.
As long as the case isn't sitting right on the carpet and the vent is filtered, there's no reason not to feed the PSU nice cool air.