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

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. 

I've found that the biggest contributors to lack of bass is either 1 speaker wired out of phase with the other, or it's a room-suckout where the position of the speakers in the room and the listening position are essentially out of phase with one another.  You might find that moving your seating position forward or backwards in the room, even a foot or so, can have a dramatic effect on bass presentation.  

Your subwoofers might be in a better position to positively affect bass reproduction at your listening position than your mains.

I would turn off the subs and focus on speaker/ room/ listening position setup first.  Once this gets optimized, then you can add in 1 subwoofer, then the 2nd subwoofer.  You'll want a helper to assist so you can stay in the listening position while they move the speakers into the proper position.  There are several resources for this including the books from Paul McGowen and the Sumiko M.A.S.T.E.R.S. setup guide that can make this process easier.

Keep in mind that once you start adding in the subs, their settings may dramatically change from what you have them set to now since they need to work with and augment the bass from your mains, not against them.  Integrating 2 subs can be quite a chore to perfect, however once completed, the entire system will come alive.

I would suggest looking at putting together a REW (Room EQ Wizard) setup and taking lots of measurements.  This can help visualize what's going on in your room and speed up the process.

I don't know about packing, but assuming your speakers have rear wall reinforcement and you haven't put them in a bad modal location your next option is some room treatment.

In a very reflective room the speakers will sound excessively bright, also known as "lacking bass."  By taming the high frequency energy in the room the bass is allowed to emerge like an old shipwreck appearing at low tide.

Thanks all. I'll give it another try. I had 803d2s before. I thought they were to bass light. I moved up to the SFs hoping that would solve the problem. It didn't. I moved up to the MLs from a Classe CA-2200. The MLs have twice the power of the CA-2200. That didn't really make a difference on the low end. I've tried every cable possible. I've moved them around to every conceivable location in my room (which is somewhat limited by the fact that it's our living room, and I can only get away with so much with my wife. Thanks again.