[RodB]

Speaker cone: yes, but the vacuum is off the back of the cone. So if your baffle is infinitely large the cone is most efficient.

Power transformer: it sounds like it has been removed from the chassis. You may be measuring the 400 volts across one side of the high voltage winding. The high voltage winding has a center tap. If you measure the end-to-end voltage out of circuit it could measure 750-780 volts. You should have three wires for the hv winding, one is the center tap. Between center tap and each end you should read 350-375 volts. The 5 volt winding is for the 5Y4 rectifier filament and the 7 volt winding is for the filaments of the rest of the tubes and pilot light. Under load the voltages should be measuring around normal or slightly higher. Mike was concerned that the 400 you measured is too low. If you report back that it was one side of the hv winding then it should be OK.