Lots of bass at walls, lack of bass in center of room/listening position

These are the ns 5000? Excellent choice. I really appreciate what Eric is suggesting. I have a 6’x8’ piece of carpet hanging on the sidewall of my listening room, it helps. One day I may replace it with vicoustics or another brand, but it showed me how important taming that 1st reflection point is. 

That is so fascinating using all different kinds of subwoofers. Is there any downsides to this? Do they have different speed or timbre? 

Speed or timbre. With a sub. Good one. Guess you missed the part above where I said:

human beings cannot even hear low bass at all, at less than a full wave.

So for proof, my first sub was a ported isobaric Talon Roc. Isobaric is two drivers mounted back to back so one removes the air pressure from the back of the other one helping it move faster. Because the Talon Khorus uses the same isobaric design and so of course the bass has to be "fast" to match. This is the way they think. 

This way of thinking is baloney. My DBA with sealed and ported, and none of the drivers as massively powerful as the one in the Roc, is way better. It sounds faster, more tuneful and articulate, in spite of what looks on paper as if it would have to be much worse.  

You have to get out of the trap of thinking of low bass the way we think of midrange and treble. They are both waves and the physics is the same but the way we hear them is quite a bit different. 

Like, midrange we can pinpoint the source in 3D space. Low bass we cannot localize at all. Treble and midrange we can hear a millisecond blip. Low bass we cannot hear less than a full wave. At all. That's like 50ms. Midrange we are very sensitive to volume and hear volume changes accurately. Low bass we hardly hear at all until it gets quite loud (hence the Loudness switch), and then once it does we are overly sensitive to volume.  

What all this adds up to is with midrange and up we need two and only two perfectly matched speakers placed precisely symmetrical to create a 3D soundstage- but we can use a whole bunch of subs of all kinds and put them just about anywhere and yet they will integrate perfectly into that 3D sound stage.

Clearly you have not chosen the right speakers for your room.  The NS5000 are WAY too big.  Adding subs to that room is insane.

Best solution... get a bigger room.

Second best... get rid of those subs and speakers and choose the right monitors for your room.

The best success I have had is using the Vandersteen speaker placement calculations for the main speakers.  This is available on their website.  I have two subs.  I have 11 acoustic panels, diffusers and traps.  I can walk around my room and the music sounds good.  It was not that way at all previously.  Several peaks and nulls. Good luck. 

+1 dznutz
...or as I suggested apply professional digital correction.