I also think sound reflecting from the walls is a likely source of the muddiness. Our brains are normally very good at disregarding 'ringing' caused by sound reflecting from hard surfaces, but when the time between when the original sound reaches our ear and the reflection(s) is very small (often because the distance between the speakers and the walls or other hard surfaces is very small) this mechanism doesn't work as well.
Maybe you could try moving the NS2 and the speakers more to the center of the room for a test? Just to see if it actually removes some of the muddiness? Placing a speaker in a corner often changes their sound (and not in a good way) - this is especially the case for low frequencies. If you want very loud, booming (and distorted) bass, place your sub-woofer in the the corner of the room :-p
Anyway, I have my monitors up a against a wall too and have some of the muddiness you mention. I don't have the room to place them anywhere else and you might have the same problem. What would help is placing something sound-absorbing behind the speakers, such as a very thick curtain, to help prevent very direct reflections from the speaker to your ear. If sound bounces of walls further away from you, the volume will be lower than the original sound and it will arrive much later (relatively speaking) than the original sound which both help your brain to filter those reflections out.
When you move to a new house (or something) you sometimes have rooms where you can clearly hear an echo; however, as you get to know that room (and how it 'sounds') you usually can't hear it anymore after a while. Whether that's because you're now used to it or because your brain is now actually able to filter it out is a good question.
Edit1: I love the music stand painted in the same color as the wall
Edit2: You could try plugging up the bass reflex port in the rear of the speaker with a sock or something (make sure you can get it out). Something like a bit of rolled up filter material (the white, soft stuff) used for cooker hoods (the thing that sucks up greasy/smelly air above the stove) might help in a bit less dramatic way than a sock, since it will still allow through some of the sound. It will definitely change the 'color' of the sound produced by the speakers though.
