adding multiple subwoofers

The crossover you are talking about is unnecessary and with your speakers actually counterproductive. The advantage of multiple subs is primarily from having multiple locations for bass. Adding four subs adds four new low bass locations. Crossing over from your mains removes two locations. This is what we call "own goal".

I’ve gone the whole route from none to one to four to now 5 subs. Was expecting you to have read myself, or Tim (noble100) or audiokinesis or perhaps even Toole or one of the white papers on subs. These would all say what I am saying.

The number of subs is so much more important than anything else that it frees you up to do four pretty much any way you want. The least expensive will be what I did, buy 4 cabs and drivers from Parts-express.com and drive them with one or two Dayton SA-1000 sub amps. The Dayton has all the flexibility you want and with four 10" drivers will get excellent DBA for well under $3k, possibly under $2k if you catch a deal and paint instead of laminate. Mine are genuine Rosewood veneer and 2 amps (really only need one) and I was still under $3k. https://systems.audiogon.com/systems/8367 

You could also get excellent results with a couple of the Tekton 4-10 subs. Or any combination of powered and active subs. That is one of the beautiful things about taking this approach, it is incredibly flexible and effective almost regardless of how you go about it.

If you split the signal to the Behringer DBX before going to the mains, it's a great idea. 

Split the signal, one pair goes into the DBX the other pair go to the main speaker amp (s), then to the mids/highs terminals on the speakers. (remove the jumpers)

Use 2 channels (left and right) to "bandpass"  the amp for the mains, onboard bass drivers. The other 4 DBX outputs can be individually programmed to do what you want. Each output requires a power amp (s). If it's a plate amp onboard your subs or passive subs with bass amps in a different location (s). 

The passive XO for the mids and highs onboard your Cornwall aren't affected. There is no added crap in the sound path. You relieve the amp of bass duty too. If it's a tube amp, you'll notice the difference right away.

Full blown OTF correction, at the seated position via laptop or the DBX console. I've been using DBX2496s for crossover design and bass management sense 2012. I don't use them for anything other than bass management in my sound system, 300hz and below. The newer units are dead quiet too.  300.00 usd (close). A good light PC sure help. I use a 39.00 silver clad, teflon weave. :-)

I also like Behringer's NU12K power amps. 3-400. A single amp will run a dozen 12" drivers over 110 db for a long long time. 

I'll put that design up against ANY BASS system design including software designed servo control.. I have 2 different types, of servo and use one. GRs. It's a good system. Nice looking, I like it..

This system is just as effective without OB design and cost 1/3 the price. Fancy can cost a lot of money.:-)

It has just as much cone control (overshoot) without someone's digital intervention and THEIR idea of what to control, you still have to write the code to make the dampening respond or NOT.

Measure away find out for yourself. The DBX has an onboard mic input, you can SEE your correction in you room in real time.. 

It's just plain cool..

Congratulations, your on the right track. The benefits of the new generation SVS subwoofers are that they include built in EQ/DSP. This allows for more individual bass EQ no matter where you place them. To get the most benefits, you need a way to visualize the bass response at the listening position. You can use any number of commercial products. I personally use an old Velodyne SMS1 sub EQ which produces bass test tones which allows me to see the bass response using a monitor. I can adjust each sub to smooth out the overall response. It’s easy for me to do now. It takes about 15 minutes to adjust 3-5 subs.