Having recently added a first version of the sub to 5200SE I would say that you probably have more options available to you given the 8 series processing than the 218 I use.

Placed dead center the 80Hz crossover sounds fantastic but inconvenient for my room, I have it now placed closer to the right speaker on the inside and it still sounds great for almost all material, only rarely do I feel more weight from that side.

I've also tried it outside the speakers as well as completely removed behind a couch but then was too distracting. My attention would constantly be pulled towards its position. If I had speakers with stronger bass initially, this may not be the case.

I'd start my testing dead center and find a good crossover frequency. From there, try moving it around to where it's most convenient.

Hope this helps.

1) 568.2mm, 7K's, 5K5HC, 4x420's, D2500, MS200, moded Oppo BDP, Epson5020 FP
2) 218, 5200SE, DSW
3) MC200 core & MS200s to HK PA4000 house distribution