Recommendation for replacement of Paradigm X-15 subwoofer

In terms of a 15" sub, this has much smaller cabinet relative to the driver than the Defiance:
https://www.hsuresearch.com/products/uls-15mk2.html

@erik_squires I will look into it. I replaced a smaller HSU with the Paradigm. I will probably stay with Paradigm as my speakers are Paradigm and my receiver is Anthem. The Prestige is a lot smaller than the X-15 as it is 12". I just don't know how it would sound going from 15" to 12". The X-15 is great, but my wife has complained about it from day 1. She does love how great everything sounds

If you have ARC you'll be fine.  12" is overkill for most living rooms anyway.