Thinks it time to upgrade my subwoofer… expert help needed

Dual 8″s nice! 😎 Pfft PB1000 isn´t available as white.

This “big baby” is going to give you most bang for your buck, see specs & dimensions here:

And the white finish DF = downfiring see here, there is no pictures of white FF but you get the idea on the links:
I think the shipping is ~30£ cause it`s large and heavy. You need to call them monday about the availability when they come back from vacation. Monolith has been the go to sub for movies for over 10years (at 500£ mark). If you want noticeable upgrade this is it. Compare the FF and DF cabinet dimensions which suits you better. Performance is same.