Don't know if this will help, but here's what I do....
I want my WAV output to be 80%. So I make Winamp's volume 80% also. Then I use the master volume control to change the volume, not Winamp. This is facilitated by my Logitech multimedia keyboard which has volume controls which change the master volume, not the wav volume. I also make the volume on all other progs 80% so I never get inconsistent sound levels... perhaps it's simple, but it works!
Also, make sure the Winamp volume slider is up high enough (main Winamp window).
Here's some screenshots to help you even more
You can also try "alt setting mode" in WaveOut Plugin Config
Try with or without "Volume Control : enable"
With "alt setting mode" only, Winamp should work independently of the Windows Volume Control
With "enable" unchecked, you'll no longer be able to control volume from within Winamp.
The combination of the two could be what you're all looking for.