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

@Audiotroy--never thought the room was that big, but you may be entirely right. I assumed it might have something to do with the fact that there is a half-wall between the living room and the kitchen. Maybe I just need the subs based on the size of the room.

@jl35--that may be it too. I really didn't expect to need subs. Or at least I was hoping that if I spent that much money I wouldn't need them. 

I guess as a side note--I usually buy used equipment because it let's me get things at a fraction of the cost that I would never be able to afford otherwise. The SFs and the MLs were both used demos from authorized dealers. With the SFs, the dealer was from out of state, so I didn't get the benefit of having someone come in and set them up. It has occurred to me that something is broken/wrong/malfunctioning, but given that the bass I am getting sounds good, my guess is that everything is working properly. It's not like the bass drivers aren't moving or producing sound. I just think there ought to be more of it.

Stupid question - is your judgement of bass based on critical listening with the chair in the middle or is bass lacking in general. It’s possible you’re sitting in a bass null.
Or the setup is just not optimal. Can you post pics in the virtual system section of your profile?

Some beautiful speakers you have there - congrats! 

I will pile on that the SF are just not pressurizing the room, which when you put it all together from your description, is quite large.  Try walking around the room and stopping here and there.  You should find places where you can actually 'feel' more bass peaks.  I know my pretty large (long) basement room has much more bass (too much) along the sides and towards the back.  Fortunately, the listening position gives me a good combo of punch and depth.  If you don't feel the bass substantially more in certain places then I suspect it's a pressurization issue.  

Sounds like you've tried moving them around as much as possible with little effect.  Just leave the subs on and do your best to optimize everything.  Then relax and enjoy your great setup!

That Classe' has a high pass setting when using a sub. Are you certain you are sending full range to the speakers when the KEF subs are not used?

Page 28 of the Classe' manual.

Good luck!