Mass-loading subwoofer

You have a $5500 subwoofer.

No need to reengineer it. Get another one if you're curious about improving overall performance of your setup as a whole.

Improve the room acoustics.

The best way to combat resonance in a subwoofer enclosure is with internal bracing. But that the job of the designer/builder. The consumer cannot add internal  bracing, as the braces eat up some of the enclosure's internal volume, which affects the sub's performance characteristics.

For a view of great internal bracing, head over the website of Salk Sound. Jim installs the Rythmik servo-feedback subwoofer system in enclosures of his own design and build, and his bracing is the best I've ever seen. Check out the illustration of the interior of the Salk/Rythmik 12 sub!

Thanks for the responses folks! 
 

I DO realize that Rel themselves dissuade against using a platform but with my current flooring it does seem to work better. 

Yes, it IS an expensive subwoofer but I paid less for it (used) than most other midline or even less brands would’ve cost. I have zero intention of changing subwoofers but merely wanting to experiment using others expertise as a starting point.
You are very correct about room acoustics though and would certainly appreciate any suggestions in that department. 

I sincerely hope that this response doesn’t come off as overly negative though and I’m merely asking for advice within the confines of my situation. 
 

Thanks so much! 

I have a 42 pound curling stone on my REL 7i. Not sure it makes any difference, but it looks good. 

… actually, I do think that it’s always good to get a completely different perspective from the way you are currently thinking so maybe I really shouldn’t limit advice at all. 
 

Thanks again!