I'd start with trying some decent strings (GHS Bass Boomers are my favs) and see if that works. If you pluck resting your thumb on the back pickup (the one farthest from the neck) try resting on the other. If you are still getting that twang go to a Guitar Center or other similar store that lets you pick up and play basses. Try plugging an Ibanez SDGR into a Fender Rumble. If you still are getting twang it's probably not your bass or any of your equipment but some thing you are doing with your hands. If that's the issue take a lesson or two and that should clear it up.
