THIS!
I've become a proponent of the Geddes technique for placing multiple subs: strategic, asymmetrical placement around the room. Especially if you have good Mains, you can easily have 5 LF sources then on 4 walls which will go a very long way to evening out the bass response in the room through the excitation of the most Room Modes possible.
Measurements are needed to perfect it, but just using the crawl and the Audyssey app did a great job for me. I'm still working on learning REW, and when I get the time, I'll be diving much deeper into that.