If you look in my locker you will see a writeup on the Aux. Air Valve and the Idle air control valve (usually called the IAC). There's a lot of photos there too. It explains the valves, where they are and how to clean them. Briefly, the Aux. air valve is only open when the car is cold. It allows extra air in while the engine is cold. After the car warms up, it closes down. As it closes, the idle speed drops. When the valve has completely closed, the engine should idle at 610 rpm.
If your problem is only when the engine is cold, it could be the Aux. air valve.
The IAC holds the engine idle at 610 rpm after warmup. It can get gunked up and be cleaned.
http://www.subaruparts.com/subaru.diag/2A/i073A-1.gif
The diagram you referenced above doesn't show either one of these valves.