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

Are your speakers new? If so, as others have suggested you may need to give them a little time to break in. If that’s not the issue, what speakers did you own before these and were you satisfied with the bass performance? if not, it’s possible that your personal preferences are such that subs will be necessary to meet your expectations.

did you buy the speakers from a dealer in your area? If so, and they are relatively new your dealer should help you with this problem. Maybe he could bring different speakers or a different amp to your room and experiment.  As others suggested, these are quality speakers, which should make enough bass, but “enough“ is subjective and you have too much money invested to be dissatisfied. If you bought them from a quality dealer, you should have a partner in solving this. Good luck.

I have Sonus Faber Cremona Ms. I also have two KEF R400B subs to enhance the bass. My amp is a Macintosh MA8950. I know my room has some issues, but I'm ok with using the subs. Sometimes, I turn them off for specific listening, but most of the time they're on. I don't see it as a problem, more of a solution. 

As  mentioned - If new, the woofer suspension will loosen up over time and start to sound as designed, but can sound restricted early on.

Double check that the polarity of the speaker wires is correct at both ends....you can even take a battery with two leads soldered on each end to check the woofers are wired correctly internally. The positive lead to positive input (plus neg to neg) should cause the woofer to surge forward...otherwise they're wired in reverse.

Bass output is relative to the volume it has to fill.  Your room is large with a large volume of space to fill.  It's possible that your room has some bass issues, and the main woofers aren't up to filling it with low frequency.  Subs can be a good, simple solution.  My speakers had plenty of bass at our old house, but I needed a sub to fill the space of our current room.

Do you have any bass controls you can turn up?  You could also try bi-amping to the woofers with a separate amp that has an independent volume control, but both of these solutions could have some complications and affect on vocals in the 80hz to 200/270hz range in addition to lower bass. 

It's not something I've experimented with, so am only suggesting it as a possible option to consider, but perhaps some sort of parametric EQ with good adjustment options below 100hz could help if other solutions aren't satisfactory to you.

It sure sounds like something is wired out of phase. Try reversing one set of speaker cables. 
 

A friend of mine, let’s call him M, has the SF Amatis and another friend of mine, P,  and I went over there recently for a listening session.

M had been tweaking his speaker placement for a good while and while the speakers completely disappeared, the center of the soundstage felt too low and the music felt flat and not as full as with our speakers (Revel Salons and F328Be speakers, respectively), even supported by dual subs. 

We had no idea why this was the case but we left a bit underwhelmed by the Amatis, not given their price, and it wasn’t for a lack of trying by M to set them up.