I'm in the camp that's willing to sacrifice sonic bliss to avoid hauling extra gear, so I've only ever gigged my SSv3 on its own. I'm generally happy with how it sounds on its own. Like others, if I'm worried it won't be enough, I use a different amplification strategy (e.g., powered PA speakers). I remember following the subwoofer discussion in the giant SSv3 as it unfolded. I was tempted to pick up that Behringer several times but ultimately didn't and haven't regretted it.
However, this thread is prompting me to plan some home experimentation with blending the SSv3 with a powered PA speaker.