German,
Yes that is the aluminum plinth base. As Dave mentions, there is no way you can damage the actual woofer or its surround. The worse that can happen is one of the spikes gets in between the grates, but I don't believe it will go as far as reaching the woofer.
Dave,
I am also considering the Sistrum platform. I know you mentioned that the speaker is still stable on it. But just how stable? I mean if someone (large) accidentally knocked into it (say on a 45 deg. anngle), will it topple over? I just have this worry about something this tall sitting on 3 feet. A big heavy amp no problem.