My Sonus Faber Amatis have weak bass even though I'm running ML 536s--800 w/ch into 4 ohms

Did I miss the part where you told us about speaker placement? How far off the wall behind them? How far apart are they? How far away do you sit from them? Are you using the Naim as a streamer and the Classe as preamp and Dac? I’m not a big fan of Naim and I’m not so sure the Classe is the right preamp for the Levinson's,, If it was me I’d look for a Levinson preamp. At then you’d know that impedance and phase are correct, Then a appropriate Streamer and Dac, I just think there’s some mismatches in your equipment and maybe speaker placement 

i have a room that is remarkably similar in terms of dimensions and the back being half wall and half open to the kitchen. In my case i have high ceilings with irrigular angles above which is overall beneficial.  Keep in mind, large rooms are always much harder to fill with Bass AND that open back wall means that there are no back wall for the bass to bounce off and get enforced.  Walk around the room.  Pay attention to see if the bass is stronger as you are near the side walls or in that one back corner.  Does it lessen as you get to that area that is open in the back.  i agree with  pkatsuleas it is likley more about the room .  i chimed in because my room is so similar.

Here’s an example

https://www.youtube.com/watch?v=M8vhVTQ1cLo

As an aside, Duality Repair, the YouTube channel linked above, is one of the best if not the best electronics repair channels on YouTube. 

Too bad he doesn't post more regularly. He sometimes goes months without a new episode.

+ "Found Nordost to be light in the bass."

It sounds like you’ve never been pleased with the bass with just main speakers in that room. When I moved into a large room, the bass completely disappeared. 2 subs are especially important in a large room and I got the best results by placing them on either side of the room, about halfway front to back but slightly staggered, and facing to the rear behind furniture. Less interference with the mains but still provided that fullness which had always been missing. Why not keep your 2 subs with your mains running full range? Then play with your sub xovers to get the right fill.

Another big improvement in bass quality and quantity came when I placed 2’x4’ acoustic treatments behind the mains and soffit traps stood up on end to the sides of the mains to reduce boundary interference. In a large room with the speakers well away from walls, the boundary interference will be deep into the bass. I used GIK acoustics which won’t break the bank. The soffit traps are placed to control boundary interference from both the mains and the subs. My bass is pretty awesome now. Good luck!