An unusual problem

but realistically you’re just going to have to turn everything down in the late hours.

Not just late hours but all the time except when theyre not in. 

Buy an Auralex Sub Dude isolation platform.  I've had one for years ...not only does it improve the sound but it eliminates many rattles associated with having a sub.  My mom had been complaining that my dad's sub was boomy and keeping her up....  he's 76 so sometimes movies are a little louder than they should be.  He says "does anybody ever say turn it down at the theater?"   Touche, I guess.  Anyway  I went over there one Saturday when he wasn't home and slipped a sub dude under the sub and ran the Anthem room correction software again and it's never sounded better.  Boominess is gone and you don't feel it in other rooms now unless it's loud.  One of the greatest accessories ever.

I have a Subdude in my bedroom.  I live in a house, but my neighbor's house is not too far away.  I think it helps a little, I never went outside with my SPL meter to see if it made a difference or not.  Eventually I just quit using the sub in there.  They never complained, but I was trying to be proactive.

There's this thing called consideration that you can get for free.  If you live in a box right next to someone in another box, don't play your music loud after 10 PM.  Headphones are a good suggestion.  That's what I do late at night.

bass waves are so long that they penetrate easily. best suggestion I read was just go to headphones at night....harmony with your neighbor is important.

I actually just got rid of an awesome REL simply because it provided a 15% gain in sound quality for me and a 75% gain in irritation for my room mate.  
I’d look into finding enjoyment in a set-up that doesn’t bother your neighbors.  Sell the sub and put that $ into upgrading the transparency, resolution  and musicality of your set-up!

Magnepans might be something to consider as I’ve heard that they don’t transmit bass through walls quite like conventional box speakers.  Magnepans are more directional so they may not sound quite as good when walking around your loft but that directionality may help you out as well since it sounds like you have glass along the entire 1 side of your listening area.

Good Luck!