I actually learned that I was incorrect in thinking my Thiel's would only accept spades. I had previously encountered speaker's whose mounting post knob had a plug in them that was easily removed. What I discovered with my Thiels is I had to completely unscrew & remove the mounting post knob & punch the plug out through the bottom.
So, I was wrong & I suspect that you have the same situation as I had.
So, I was wrong & I suspect that you have the same situation as I had.