| umm try using a pick and hitting the strings hard?
it actually sounds to me like the bass is using some light filtering, not wah. And there is a gate applied to stop it from ringing through.
picks just give a better attack from the strings, doesn't matter how hard or soft you hit them, you are still going to have that characteristic trebly bass sound that you get from using picks. I've never heard of a wah type sound coming from the strings just from hitting them hard with a pick. If its anything simple, its a post production trick. | 
