Hahaha - I was literally just thinking deeply about this with great anxiety as I have for the last two months (context: I just started playing tuba again 2 months ago). I live in an old pre-war NYC apt building (brownstone is a nice word for it).
I'm in the market for a Silent Brass system (you should get one), but here are some practical considerations:
If you don't have one already, get a LARGE (the kind that properly takes up almost the entire room; save 1.5 - 2ft from the wall). Do this even if you have carpet flooring. You can go cheap and still end up with something fancy-looking:
https://www.amazon.com/gp/product/B01DW ... UTF8&psc=1" target="_blank" target="_blank The extra layer of carpet will absorb sound and you water valve deposits. To the later point, this will ensure you get your security deposit back!
Any plush furniture (couches and pillows) will also act as a natural sound dampener. Tapestries and thick curtains also help.
What I've mentioned so far is practical advice that also will also spruce up your apartment. If you're into feng shui, or just appreciate good design (but don't have the eye for it), consult a friend that does, or skim some DIY interior design blogs for advice of buying the correct rug/drape size/colors etc.
And the most important thing: TALK to you neighbors, especially those above, or directly beside your practice room. When I bought my horn, I worked out a deal with my neighbors: no practicing after 9 PM on weeknights.
The only reason I'm still in the market for a silent brass system is so I can practice first thing in the morning. Good luck!