I haven't read all the repsonses but some were right. The connection of your speakers to the floor is the number one culprrit. Sound travels faster through solid objects than air. You can try Iso Acoustics for that issue or build you own isolation platforms for the speakers
Yes interior acoustics will have benefit to tamming a reverberant environment which neighbours can hear, but it won't stop most of what they hear. Interior acoustics is mostly for your listening benefit.
REW is a tool, it does NOT tell you how to deal with acoustic issues, only experience & knowledge does. I've been designing rooms for over 2 decades and it has become a key offering in my audio business. Apart from buying some used acoustic treaments, there's no inexpensive way of getting a room to sound great.
effective materials cost money today even if you DIY. Learn about acoustics and what really works from a physics stand point, there are too many pretenders out there. Vicoustic seem to have the best price performance and WAF
Gary / audio by di tomasso .ca