Search "Speaking Tubes, Romans" and you will find that although you have me outnumbered, to state that there were no speaking tubes when the song was written is to state that the song is pre-Roman.
Could be!

Then give some thought to the line, as it is used in the song. Does it really make sense in that context?
"Blow the man down" would certainly be a command bellowed out by a skipper seeking more hands up on deck and one which would be revered by all but "Kill the man"? It doesn't fit.
Sorry guys. This one, I win.
Cheers
Jim