Oh, this is awesome. I've been resorting to screwing with every programs volume.
When installing, did you select the microphone device as well? Just using 'preamp:' in the config file affects any devices you selected.
If you want to adjust volume for each device independently, make sure you have the correct devices selected in E. APO, then try a config something what I'm using:
Device: Corsair Vengeance 1500 Headset
Preamp: -8db
Device: Microphone
Preamp: +20db
Found the how-to on the project's SF wiki.