Downward firing speaker port and domestic issues

In my experience, it matters not at all where or if there is a port or a passive or if a sealed box.  Bass goes everywhere, if you have it.  Your best bet is to either luck out with an elderly hearing impaired neighbor, or to get a really nice pair of LF challenged compact monitors and a sweet EL84 amp to drive them.  Black Ice F11 for example, with a pair of LS50s or similar.  Keep the volume down and enjoy.

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.

Decoupling your loudspeakers from the floor will help both sound transmission and the sound of your loudspeakers, although you may still need to moderate your volume when your neighbor is home. I recommend the Auralex iso platforms from personal experience. The "Subdude" models will work fine for full range loudspeakers and are not expensive.

I hope this helps!

To address this issue, we need to address the topic of "bass". Your sitting comfortably at home and someone drives by with a rather loud and obnoxious car stereo. The bass overpowers everything in your environment, and large objects start moving aroiund the room. Althought the car is "decoupled" from your space, the bass escapes it’s environment and finds it’s way into yours. The car is relatively "sealed", yet the bass finds a way out. It travels a hundred feet or so, penetrates your siding (or brick, or stone), sheathing, 4"-6" of insulation, 1/2-5/8" of drywall and rattles your brain. Long wavelengths with great intensity are difficult to constrain.

The second issue IS coupling to your room. Sympathic resonances caused by the vibration of your speakers couple with your subfloor, finds their way thru the insulation below, and through the ceiling material of the lower apartment. So, you’re actually dealing with two highly related actions here. YES, coupling will address one issue, but not all issues because high energy, long wavelengths are highly persistent.

The better question(s) are not so much WHERE the bass port is facing, but rather how low the tuned frequency is. Bass penetration is highly relevant to wave lenght. And, how loud you play.

As @ghdprentice suggested, getting to know your neighbors is the key here. You might ask them to help you discover the threshold of pain for them in THEIR apartment via your audio system, and establsh guidelines for behavior. You might discover that your music is not as intrusive as you thought.

I'm in the same situation.  I live in a rented apartment with neighbors all around.  I'd avoid a down-firing floorstander.  You might be better off with front-firing standmounted speakers decoupled from the floor.  If the room and the speaker allows, move them a little away from rear and side walls.  With the right speakers you could have very good sound at moderate to low volumes.  When you're listening you don't want to have to be concerned about whether you're disturbing your neighbors.