No, they don't plug into each other. The manual, if read literally, suggests that:
- The amp be connected to a 20 amp circuit
- Don't connect multiple amplifiers, or other heavy power devices, to the same circuit
Clearly, this is boilerplate copied from the 770 manual. Nobody has seven separate 20-amp circuits to their home theater room. A few people might have two.
In practical terms, these amps are going to get plugged into power strips or other multi-outlet devices. As long as they can handle the capacity (theoretically 15 amps for each 3 amplifiers, in practice a lot less) it should be OK.
I have 3 M-blocks (and the rest of my home theatre system) on a single 15-amp drop (since that's all the room has, and I can't install more because I rent) and it hasn't blown any fuses yet. The lights do dim a little if I turn everything on at once.