The Sonos Sub is not designed to be placed underground. What sub are you referring to?

I have the 6.1 magnolia outdoor speaker system with the in ground sub 

I haven’t put it in the ground yet 

I’m assuming you are referring to the Sonance 6.1 outdoor speaker system. The sub in that system is designed to be installed in the ground. If you want more bass, place the sub in the ground near a wall or other solid structure to reinforce the low bass frequencies.

I will try that 

thank you

Not sure if Sonance has a forum but they are different than Sonos.

Sonos does sell a select few customized Sonance products.

https://www.sonance.com/

https://www.sonos.com/