Cornwall IV and subwoofers

I am. I have the Cornwalls on low (6") amp stands to get them slightly off the floor in my 13 x 22 listening room. I'm running a Pass 250.8 amp, PS audio DSD JR and Pass XP10 pre amp. I have a REL s510/SHO sub on high level inputs off the Pass amp and a pair of SVS PS3000 subs (sealed box, not ported) running off the pre amp outs on the XP10. I like them because of the mobile app interface and the ability to tune them via the app. I run them all so they come in around 60 hz or so and at a low volume. They all integrate really well. Te Cornwalls are great but lack the very deep bottom end. I also use Roon and Quboz and like Roon really mostly for it's parametric EQ functions. The Cornwall do have a tendency to have a bass boost in the 125k hz area or so, which I trim out with a deep notch filter adjustment on the Roon EQ. overall I'm pretty happy with the Cornwall - they're huge however- even bigger in person that what the measurements suggest! 

For me, and again for me... I have Heresy IV's with an SVS PB2000 pro. It's everything I need. The Heresy's with my Mac 240 are a sweet combination. The SVS gives every recording an easy weight and presence. I really like the SVS mobile app and pre-sets, really lets me dial everything in. Just my two bits..

I have the Cornwall 4’s in a 14x30 room with an SVS SB3000 sub. The sub fills in the bottom octave nicely. I never thought a sub was necessary until I actually tried it. Big improvement. 

Piggy-backing on what @jbhiller said very well, I use the Cornwall IVs with a pair of REL t7i's, speaker level from a Pass INT-60. The RELs have 8" active, 10" down-firing passive. They're small enough that the OP might be able to accommodate a pair. I've tried to cross them around 30-35, although REL doesn't make it easy to be precise in that regard. As jb said, they're not really necessary, but I like to think they add just a touch to the Cornwalls' palpable sense of presence.

I have a pair of Klipsch corner horns, just the bass section.  The go to about 40 Hz and at 106 dB/1w are effective for most listening.  I would not add a sub, but built one anyway. 

I call the Klipsch "subs" because that seems to be the vernacular. 

I do have a sub-sub, which is a 14' transmission line with 4 15s.  Three of those.  TLs are narrow band but efficient.  I can't measure the SPL but it should be flat to 4 Hz and judging from vertical floor movement, about 5.2 Richter.  Good for rocket launches, end of world movies and such.  Maybe some music.