I’ve had Rel for years. They do much more than just augment bass. They make the main speakers image much larger than they do without the sub. This is why I chose 2 Sumiko S-10 which is a Rel clone of the S-5. Sumiko was the distributor of Rel for many years. Thus they knew the design and how to clone it. You may be able to find an s-5 or S-9 on closeout somewhere since they are no longer made.
Here’s an S-5. Two may work or look for S-9. BTW I have no affiliation with either company
https://salon1audio.com/product/sumiko-subwoofer/
Here’s an S-5. Two may work or look for S-9. BTW I have no affiliation with either company
https://salon1audio.com/product/sumiko-subwoofer/