perhaps the treatments negate the need for one at all.
They sure do reduce the need, yes. By reducing the mid and treble energy in the room they can restore the bass/treble balance.
Even if adjustments need to be made on the sub, such as more futzing with the x-over, it seems to be a lot of work for a small return.
Often true. I've never been against subs, but the road to great sounding subs is fraught with peril.