What worked for me was carefully explaining that the front of the speakers are very delicate, and that if he touched them they could break. And that would "make daddy very sad" and we wouldn't be able to listen to Steely Dan anymore (he's a lifelong fan).
He understood almost right away and I haven't even had one close call. That said,
I wouldn't ever leave all the drivers exposed. Toys go flying in all directions without aim - there's no protecting against that. The
foam grilles add a layer of protection.
It also seemed to help that I showed him parts that he
could touch, namely the outriggers and the side of the cabinets. I had also brought him down to watch as I worked on them over the course of many months. I think that helped him to understand that they were important to me.