I have the rubber feet on, as I have hardwood floors, so that's no problem. I'll have to try tipping them again myself, although my wife says that that's not a valid test, as I am not <3 feet and suicidal.
The speakers are pretty smooth and not much for them to grab onto should the child start to topple over and reach out to stop the fall.
I found I just had to make sure when ever they got near the speakers, the first few hundred times, I always told them not to touch and please stay away, eventually it sinks in.
I know I have seen some people building stub walls around the entertainment units/speakers to keep the kids at bay.