Underneath the passenger seat.

[Ghost]

I own an Audi A3 S line 8V (facelift). There is a box underneath the passenger seat with vents on either side and on the front. I was wondering if anyone knows what it is. I do have the optional Harmon Kardon sound system, maybe it's something to do with that? If anyone has the answer could you let me know. 

 

Thank You in advance

 

 

[Steve Q]

It might be the sub or amp.