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

Obviously there is something going on. 

It does sound like phase thing, this is easy to test.

Play some bass, move the balance full left, listen, then full right, does the bass go up at each extreme L/R? If so, it's a phase issue. 

Next thing would be is everything hooked up properly? Would suggest pulling everything apart, cleaning all contacts, hook it all back tougher. 

Of course speaker placement, this will make the biggest difference. Sadly best bass is usually not the best tremble on placement. You may be sitting in a null point, will just never get good bass. Again, play some bass, walk around the room. Does the bass go up and down? If you move from your listing spot is there more bass? If so, null, you got to move out of it. 

You said you have a wall of glass? Need to treat the windows, glass walls are about the worst you can have in audio. 

My Dali Epicon 8's sounded great without subs, but much better with a pair of REL subs...many "high priced" speakers sound much better with a pair of high quality, well matched and set up subs ...

OP,

I think we are zeroing in on the problem. You are looking for crazy high bass.

B&W are know to be very bass heavy. And you you owned B&W803’s and did not think they had enough bass. So, what is going on here sounds like you want a very bass heavy system. If you want accuracy down to the bass and then a big kick, you are going to have to use subwoofers. and adjust them for the kick you are missing. Amati’s are made to sound natural and accurate down to around 28 hz or so and they sound that way,

I got rid of my subwoofers when I got my Amati Traditional. Don’t miss them at all, plenty of bass, Note, Sonus Faber makes three levels of subwoofers. That Gravitas are absolutely beautiful and match the Amati.

Alternatively you could go with the current top of the line B&W 800 speakers and massive MacIntosh monoblocks. These really rock and are very bass heavy. On the other hand MacIntosh is details / high frequency light... so the might create a problem for you.

@torke 

I love great fast tight bass so I can empathize. There is a nonzero likelihood that your speakers work perfectly as designed, but they're just not right for you.

My humble suggestion would be to audition Revel Salon 2 and see if you are pleased by their sound.

Nothing else in your audio chain has the potential to cause a massive bass deficiency, so I think comparative speaker listening is the fastest way out of this predicament.

Happy listening is around the corner. Good luck and keep us posted!

the OP does say that the speakers sound "great" and with subs "comes alive" .... maybe all is well, except he didn't expect to "need" subs ...