You own a DSM, thats a learning experience in it self. If you still own one, you apparently didn't learn anything...jk.
I'm just trying to help the kid out. WRX's are picky about shit like that. It's one of those things you need to go with that will work well on that specific car, not how it looks/sounds/etc. They don't like to behave with certain blow off valves, simple as that. A little research about the car on nasioc, or any other subaru specific forum will tell you that.