There is some solid advise given, and some a bit drastic or too much too soon.
I had a loft studio space in an old textile building in MA years ago, so I have some experience. Keep your speakers, do all the normal furnishings you are thinking: rugs, plants, things to break up the directed sound waves, and a couple ACTUAL bass traps to start. Like other mentioned, go introduce yourself to your neighbors, take some cookies or something from a great local place. DO NOT MENTION YOUR SPEAKERS AT FIRST VISIT. I had issues with one neighbor that I realized all started because I began my introductions with them by saying "let me know if you can hear my system": within a week, I was getting constant calls and door bangs for just the TV being on LOW volume in the background. SO -say hi, maybe see how they have their spaces set up, maybe one of them has a great sound system too! You can likely tell their personalities and possible issues from that first visit.
The amount of cash that the avg audiophile spent on equipment... i am pretty sure he could have afforded the downpayment on a standalone house, end his subservience to the landlord and neighbors.
Two houses ago, i had a stand alone house and a rig in the basement. It never was a problem with one neighbor, buy, my other neighbor kept complaining whenever my music played. I would stand outside my house pause/play the music and see no difference on my meter. The sheer noise floor outside, i.e., dogs barking, the wind howling etc would easily drown the slightest hint of any music playing. But, somehow, it went through my basement, got through his walls and was heard above the noise floor of his fridge, ac, etc. Some neighbors are just full of sht....His lousy dogs barked all day long like cracked mofos and that never bothered him (it wasn’t loud at all for him!)...it was the faintest hint of music that got to that psychotic individual somehow. I can only imagine how it is for guys living in apartments.
Fast forward 15 years and i have a 8 acre lot. I had to endure additional wallet abuse and buy the land around my primary residence just to keep the psychotic pests and their noise complaints away. If my music bothered you 8 acres away, i ’ll just keep calling the nut house on ya! 😁