Subs for SF Amati

Um, go with 15" and be done with this.

It's an absolute myth that big drivers are slow.  They are not.  What happens is that integration is a PITA no matter what you do.

Take a look at the AM Acoustics room mode simulator and try to keep your listening location, speakers and sub out of the lowest modes. 

Measure and clip any peaks so that you can bring your sub up enough. 

Plug your main speakers if they are ported, this will help you raise the crossover point, minimize IM distortion on your mains and let the subs do what they do best: Amplify your dynamic range.

+1 for JL Audio, but you pay a lot for the excellent room correction. 

and get the JL Audio CR-1, it makes integration a breeze - and with endless flexibility for current and future main speakers...

@paradisecom 1++

Subwoofers can not beat the laws of physics. Play a 20 Hz test tone at moderate volume and place your hand on the sub's enclosure. It will be shaking./ Newton's third law. Any shaking or vibration of the enclosure = distortion. 

Balanced Force subwoofers place identical drivers on opposite sides of the enclosure operating in phase. The forces generated cancel out and the enclosure remains fixed in space. Any vibration left is cabinet resonance, another significant problem with subwoofers. Balanced Force subs are currently made by Martin Logan, KEF, Magico and me. Me is way too expensive:-) All other subwoofers are fatally flawed including Rels and JL Audio. Eventually they will all come around and use balanced force designs. 

.......giving this some thought and as a previous owner of the Sonus Faber Amati Traditions, instead of adding two subs why not look into moving up to the the new Amati's G5's ? It might be money better spent and I have '' heard '' that there is a nice improvement in the bass on the G5's. Full disclosure and my opinion for what it is worth, unless you use your system for movies , TV I have never been a big fan of subs but rather looked at taking the money spent on subs and buying a more '' full range '' speaker. You should have no problem selling those Amati's. Enjoy ......