Glasspack or cherrybomb is a muffler that has packed fiberglass that surrounds the exhaust channel. They reduce back pressure, but are not effective at muffling noise. So you can preserve more of the engines power while sounding louder then stock muffler.
Not sure what setup your truck is like, but I would love to switch mine to a Borla cat back duel exhaust, as it is single exhaust right now. This may not get it as loud as a glasspack, but ads some of that "throaty" sound. It may not be as cheap as a glasspack, but I believe will gain more power.