Not sure whether a down firing port will bother your neighbor more or less than a front or rear facing port or a sealed box—probably makes little difference relative to the overall bass output of the speaker. The more air the speaker moves the more trouble your gonna have with your neighbor. Decoupling from the floor will help so tweaks of this sort are worth exploring. FWIW I have never heard a speaker with a down firing port that provided quality bass—muddy, loose, slow, lots of overhang.
