That's why places like Radio Shack stay in business! They keep selling the parts to people experimenting with things!
As far as a potentiometer that actually changes the resistance from one value upto another depending on the range. This also effects the current which then effects the signal strength to a degree. Plan on buying a box full for sure for trying that out.
(also the votage rating if too high is intended for something else and usually won't work.)
One thing I made sure of when looking at any new pair of headphones was to see a separate volume control on the cord or one of the ear pieces for corded or cordless models rather then running into the too loud or too low problems.
Your options if no volume control or level mixer is found in the card's own software or updates would be swapping cards or headphones that have a separate control. That's the unfortunate circumstance.
But it seems unlikely that the application part wouldn't have it in some form. And that is precisely what it does have according to the images on the TB support page.
http://www.turtlebeach.com/support/index.php?View=entry&EntryID=520318781
That shows the Windows mixer right next to the Audio Advantage mixer for that model.