Physics is likely working against you. Listener and subwoofer placement in the room is important. Moving the listener and/or subwoofer makes a big difference.

Have you tried adjusting the Sub’s level up a bit?

What content are you trying, does it have a strong low end that the Sub would be reproducing?

I am very familiar with acoustics and sub levels I consider myself somewhat of an audiophile I managed to fix the problem by hard resetting the router. I have to say that is not an obvious fix and the logic defies!!!! But the difference is massive.

Not an obvious fix, but a clean restart does often clear up issues! Glad you’ve got it sorted though. 

If resetting the router helped you may be able to prevent this from happening again by setting static/reserved IP addresses for your Sonos from the router’s DHCP settings page.