Hi, @n80. Based on your description, that’s pretty much the same way I have subs connected in my main system. The line stage connects to the basic amps powering the main speakers using XLR outputs and I run the subs via additional outputs (RCA type) on the line stage. I did this deliberately for a couple reasons- one, I did not want to run my main system through the crossover in the subs; I also wanted to run DSP on the subs (simple, so that those RCA outputs go into the DSP unit, which in turn connects to the subs).
There are a bunch of threads here about swarm subs- that is, using multiple subs to even out response. If you haven’t bought a subwoofer or two yet, look for the ability to change the phase of the subs- it will help in integrating them with your main speaker system. I use horns which have an integrated conventional woofer system, and a combination of physical positioning, crossover point -usually adjustable, as well as slope- the degree of roll off of the subs, along with that phase control, which is variable- allowed the mating of the subwoofers to the main speakers (which are running full range) pretty seamless. I’m running a pair of 15" subs that roll off at 55hz on a 24db/octave slope so the subs are really just doing deep bass, which is what the main speakers needed. Good luck, time consuming to dial in, my wife helped tweak the settings while I sat in the sweet spot, but the outcome was a significant improvement on a very good system that was somewhat bass shy.