Advise required : Subwoofer for Polk Speakers

[joshi4087]

HI members,

Greetings to all.

I am using Polk db6501 marine speakers in my vento. i am planing to add a subwoofer. Request your suggestion and the best suitable subwoofer.
My head unit is Pioneer.

Also, recently seen a new subwoofer which we can fix below the seat (the size is almost same like amp). Is this s effective.

Pls advice.

thanks in advance.

joshi