I always have used subs in conjunction with crossovers, including LP for subs and HP for mains. Given level adjustments and room compensation, I can always make the subs undetectable (except for more solid bass).
Recently, I added 2 more subs. After many iterations I’m highly pleased with it. The bass FR is even at the sweet spot and now at other listening spots as well. When I have guests and I take a different seat, I now hear what they are hearing.
Four hefty subs would be too much for my 20 x 13 ft room without all the DSP.
I use Anthem’s ARC, which required a little tweaking with my 2 JL Audio F112, but considerably more after adding two SVS SB-3000. Once I reduced the LP filter for the subs from 130 Hz to 105 Hz, the sound was excellent. HP to the mains is 65 Hz. I don’t know why ARC sets them different from one another, but with subs that rolled off quicker on top, that worked fine; once I added the SVS, which play higher, it didn’t.