Dee Dee,
I've been trying to figure out what could be wrong with your metering system if you're consistently getting under exposed images in Av. I can understand what would happen if you were in Av and had your ISO set too high so that the camera wanted a shutter speed of 1/16000 but could only bottom out at 1/8000 because that's your fastest shutter speed. But that would result in over exposure. The images would be too light.

Is it possible you have some exposure compensation dialed in on your camera? Is the exposure bar in the middle of the exposure line when you're in P, Av or Tv mode? Sometimes this setting gets moved inadvertently and it takes some poking around to find the problem. Just a thought.

Other than that, I'd check the meter by shooting a wall under constant lighting in Program mode and recording the exposure. Then I'd shoot it in Av and Tv and check whether the EV is the same for both of them. If all three match, then it isn't your exposure system.

For my action shots, I actually shoot in Program mode on bright days outdoors. I get the optimum combination of F-stop and shutter speed. Indoors, I have to control the aperture more so I shoot Av.

Now, my stuff tends not to be as creative as yours because I'm not trying to selectively focus on my subjects (I can create the same effect later in PS) and I'm always trying to shoot at my lenses' sweet spots (which tend to be F8) if possible.

If you're not getting the effects you're looking for with the camera, it's either the camera or it's your technique:). Now, wasn't that helpful?!!!

Jim