Add a subwoofer?

You obviously are not familiar with Cornwalls which are capable of prodigious output. They will swamp two 12" drivers. I many systems subwoofers do more harm than good. If the OP is a critical listener and continues along the path he is on he will be very disappointed.

@mijostyn First, the OP has already said he’s not satisfied with the bass he’s getting and is looking for improvement, so clearly their bass output is not all that prodigious in his room. Second, there’s no way a pair of 12” subs that go down to 20Hz -3dB will not add meaningful information that the Cornwalls are simply not physically capable of — that the subs’ drivers are not 15” is determinative of nothing.  And third, he can try the SVS subs with absolutely no risk, and I’d bet big $$$ he would not be disappointed and would not return them.  There are precious few speakers (if any) that wouldn’t get at least some benefit from adding two good 12” subs just to better manage in-room bass issues if nothing else, and the Cornwalls are not “magic speakers” that somehow defy physics and logic.

@soix Hey, I am all in favor of subwoofers and SVS makes a high value 15" subwoofer. The Cornwalls go very loud, just not very deep. The deep has to go as loud as the not so deep. A Pair of 12" drivers will have to work too hard to keep up creating more distortion. Cornwalls require 2 15" drivers or 4 12" drivers. I use 8 12" drivers and my system does not go near as loud as Cornwalls. The reason I use 12" drivers is to keep the size of the enclosure down. Each enclosure gets two drivers. The amount of gain required to give you the visceral experience of a live performance is formidable. 

I use a Rythmic 15 in sealed sub. I high pass the mains at 50 Hz and set the sub crossover to 50 Hz. May old ProAc monitors are "rated" to 27 Hz at  -3db. The integration is seamless and the bass is tight and "musical." IME it is challenging to integrate a sub w/o high passing the mains. The Rythmic gives you a lot of control including a parametric EQ. If you have the $$ two may be great with your Cornwalls. 

@george53 You have a pretty big room so I would suggest no smaller then a 12" and probably two. You haven't mention how you have the room layout and any room treatment. Integration is the key if you know how to mange it.

Thanks to all who offered their thoughts on this.  I went with one SVS SB 1000.  Easy to tinker with settings through the app.  Huge positive change in my system.  I can see where 2 would be peachy, but down the road maybe.   Don’t think I could go back from this.   Kind of magic .  Box disappears but music gets bigger.  I felt compelled to post this, as I’ve read posts/questions here, with no follow up and I wonder how things turned out.  So thanks for the encouragement.