One of the easiest things to do to upgrade your playback space is to add acoustic panels around your listening area to create a Reflection Free Zone. You can buy these panels from any number of vendors, or if you feel like swinging a staple gun, build your own for $150 or so by following the plans from my Studio Builder’s Handbook (written with engineer Dennis Moody).
These plans use the studio standard compressed fiberglass (Owens Corning 703) or the better choice, Roxul Rockwool. If you really want to do it cheaply, there’s another insulator that you can use that’s a very common household item that you can get much more cheaply. Check out the video below (which also looks at how other common materials fare as acoustic material).
Now the downside of what you’ll find is that this material (you’ll have to watch the video to find out what it is) works over a pretty narrow bandwidth, so if you’re serious about this, fiberglass or rockwool is still the preference. Also, the material isn’t fire retardant like fiberglass or rockwool. Sure can be cheap though.
You can read more from The Studio Builder’s Handbook and my other books on the excerpt section of bobbyowsinski.com.