Add a subwoofer?

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.

Why not?????