Ok, thanks all for your answers, and beachycove for the picture. I googled a little bit more, and found (please correct me if I'm wrong) that the speakers originally bundled with the Cube were not the Apple Pro Speakers, though of a very similar (and confusing!) design. With the Cube came spherical transparent speakers with three circular holes, yet the Apple Pro Speakers later bundled with the iMac G4 and some PowerMac G4 were semi-spherical and didn't have the three holes (the speakers' membranes are exposed). Those used a special mini-jack that carries audio and power.
The ones I just bought are the latter. One can use them on any Mac using a small device from Griffin (can't remember the name) that separates the cable into a standard audio mini-jack and a FireWire connector used to draw power. I thought I had one of those new and boxed ready to be used, but I can't find it. It looks like I'll try and sell those speakers quickly again, and maybe someday find the famous Harman/Kardo Soundsticks and iSub, which are (and sound!) beautiful.
The ones I just bought are the latter. One can use them on any Mac using a small device from Griffin (can't remember the name) that separates the cable into a standard audio mini-jack and a FireWire connector used to draw power. I thought I had one of those new and boxed ready to be used, but I can't find it. It looks like I'll try and sell those speakers quickly again, and maybe someday find the famous Harman/Kardo Soundsticks and iSub, which are (and sound!) beautiful.