Having fun building your own bass traps and acoustic treatments would depend if you the type of person that likes to take the DIY route, and save a little money. GIK is a great company, but you can build traps that will perform just as good in my opinion. GIK, Realtraps, and others use the same materials that many of the DIY folks use. As long as you make something comparable in design and specifications, it should perform as good. The more bass traps you have in a room in correct locations, the flatter your response from 20hz to 300hz will improve. The peaks will be tamed and the dips will be improved throughout the room.
First reflection treatments are different. They primarily help with mid/high frequencies and are placed on side walls/ceilings where the first reflection points of your mains/center speakers reside. This helps in reducing echos and having the same sound arrive at your ears at different intervals, which reduce clarity and effect soundstage.
Again, results depend on how many bass traps/treatments you include in your room, room size, thickness of the panels, and material used.
Furniture and other items in the room will help, unless they are reflective in nature. Bass collects in wall/wall corners, ceiling/wall corners, and floor/wall corners. Those can only be teated with bass traps that straddle the locations, or superchunks.
I would recommend you spend some time reading
This articleIf you search for my superchunks thread on this forum, I have some pictures throughout.
