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

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!

@torke

More watts aren’t always the answer.  I have the SF Amati G5’s and have PS Audio BHK 300 amplifiers that put out about 600 watts @ 4 ohms and I get amazing bass.with them being 91db efficient I rarely turn the volume past 30 and if I go to 40+ the whole house starts vibrating.
The speakers are 4’ from the front of the speaker to the front wall and 3’ from the side walls. They are 7’6” apart and my listening position is 7’6” from them.Oh ant they are toed in so they aim at my shoulders.

Having said all that, I can honestly say that it’s not the speakers. I have no knowledge of your amps or your room, but your room and placement make a huge difference in the sound of these speakers. What’s your room size? What is your floor made of? Do you have carpet? When you clap your hands near your listening position can you hear any echo? Same question but near your speakers? How far apart are your speakers? How far are you from your speakers? Before you got the SF speakers, what did you have? Did they seem lite in the bass also? Have you double checked that the speaker cables are connected correctly?
 

All the best.

@torke 

I was just reading about your amps and I have another question or six.  What are they plugged into? Are they sharing the same circuit? Is it a 15 or a 20 amp circuit? Are you using a power conditioner? Are you using the stock power cords?

All the best.