There are no hard rules for subwoofer cabinet placement. It is recommended to consult an acoustician, and/or test where the subwoofer(s) performs best in the room. For subwoofer cabinets used for the LFE channel, a good starting point is typically at the front wall, halfway between center and left or center and right. 

When multiple subwoofer cabinets are used, two approaches can be taken:

1. The cabinets can be mounted next to each other to optimize their acoustic summation and headroom.
2. In rooms where strong modal behavior is experienced (i.e., large variations in measured output across the room), the cabinets can instead be spaced across the front wall so to temper room mode dominance.

When multiple subwoofers are solely used for bass management, we recommend placing them close to the speaker(s) they are managing (e.g., towards the rear of the room if they are managing the rear surround speakers).

For more information, see How to Design a Dolby Atmos Mix Room.