In reply to:

Now if I were to go by graphs alone, then the 80’s should be more than adequate for me



The graphs I saw at soundstage showed the 80s to begin bass rolloff after 60 hz.

There's a lot of bass below 60 hz that the M80s can't play as loud as a sub could, so the graphs do indicate wheather or not one would want a sub with their speakers, based on how much they want that last octave of bass there.