Can neighbors hear my subwoofer — and what helps?

Bass couples into the building structure, so yes — floors and shared walls carry it further than you think. What helps, in order of effect: lower the sub’s level (the honest fix), high-pass nothing below what you need, decouple the sub from the floor with an isolation platform, keep it away from shared walls, and prefer sealed alignments in small rooms. What does not help: pointing it a different direction — sub frequencies are omnidirectional.