Good bass traps will make a bit of difference to how much bass is getting to your neighbour as they soak up some of what is in the room, but I don't think they will stop it a great deal. They will mean however, that you can place the speakers closer to the walls because the walls will be reflecting less energy. ..I'm sure someone asked about that.

Sound proofing is about isolation as much as absorption. To stop the sound getting to the neighbours you want to isolate your room. A common method is to build a room within a room but you might not want to go to that length.

To make a current wall more sound-isolating you can build a second skin with sheets of plasterboard and space them away from the current wall by about 4 inches. Behind this the gap should be filled with fibreglass sheets, but make sure neither walls touch this because you don't want vibration transfer so there needs to be a small gap on either side. After that you need to seal all gaps with bathroom sealant or similar so that the new wall is air-tight, this means right along the ceiling, floor, walls and gaps between the plasterboard sheets. At the end you should have a pretty good wall! Please note this really only works on a concrete floor because on a wooden one the sound will transfer through the floor by vibration – that's why we normally build an entire room within a room suspended on rubber mountings rather than just a single wall.