I had two JL Audio 110E subs, the JL Audio CR-1 crossover, and a DSpeaker room correction (which I used on the subs).
To answer the question:
1. Easy.
2. Hard.
3. Impossible.
First, it was hard in the sense of trying to find a place to put two more bloody speakers in my room. And then the hassle of getting AC to the subwoofers.
But the CR-1 crossover is brilliant. Simple, fairly intuitive, made for folks like me who don't want to make a second career out of integrating subwoofers (I can not BELIEVE how much time some people spend on integrating subwoofers, the endless room measurements, the tweaking...)
So it was actually easy to get an initial good blend of the subs with my main Thiel speakers. But...being very picky about integration...it was hard to get a completely seamless integration. If I spent as much time constantly making adjustments like some subwoofers enthusiasts do, I may well have got a more seamless coherent blend (the Dspeaker did help too).
Impossible in the sense that even when the integration sounded very close to seamless, it still changed the tone/timbre of my speakers. Always. And I'm super, super picky about that. The voice or timbre of a speaker is my number one priority, and if that changes, it ain't the speaker I bought. The subwoofer always altered the tone somewhat, away from what I liked.
It did even out the bass!. But I actually preferred the punch and coherence of the speakers when the subs were turned off. The CR-1 crossover makes it super easy to flick a switch between the speakers run full range and then with the subwoofers integrated.
Anyway, sold all the subwoofer stuff and never looked back.