If you have a nice system why do you really need room treatments?

Have you ever listened in an acoustically ideal room? Think about why some people use headphones exclusively. A properly treated room will make your speakers sound like giant headphones. You'll hear the sound field embedded in the recording instead of the one in your room. 

There are two aspects to room treatment. One is to improve the balance of the frequency response and minimize room modes, mostly on the low end. This generally requires both diffusion and absorption. Not all rooms need this, and it can be addressed in different ways.

The other, and more important one is to reduce room reverberations in the mid and high frequencies that smear the sound, and add the sound of your room to the sound coming out of your speakers. This can be accomplished with absorbent materials. Most rooms can benefit from this and can take your sound quality to another level. It doesn't have to be complicated. Furniture, carpet, can help but placing 4-6" deep absorbent panels strategically works wonders. And they can found with all sorts of decorative looks. 

Very few residential rooms have any positive "room sound quality". The improvements with room treatment has as much to do with reducing "room sound" reverberations that distort the sound coming out of the speakers. This is really more important than modest deviations in frequency response, and will improve the sound quality of any room.