It's important to remember that it's more then just wattage that makes this thing work. You also need current.

Crudely put, current allows the amplifier to better control the motion of the loudspeaker. The higher the current, the more likely the speaker is going to do exactly what the amplifier asks it to do. With multiple speakers, you need a higher current amplifier.

And no, not every manufacturer provides accurate current specifications, if they provide them at all.