65 watts should give you enough power to easily fill a small gym. In an outdoor arena it should be OK depending on the size of the crowd and the area you need to cover. If your playing to a few hundred folks in say a smallish park, your fine. If you have 3000 people in an ares the size of a football stadium, then you may be pushing it. If you have access to a PA system I'd mic your amp and drive through the PA.
The more important question though is.. does your amp go to 11??? If it does then you have nothing to worry about.
