??? The Sub Dudes, or something else?
Not sure how widespread they are but a well known audiophile platform that come in different sizes, finishes and ranges
Company started in 1993, search for Audiophile Base platform.
It could just be that what you’re hearing is the difference in distance to the floor. Even on a platform (I was using a Sub Dude platform), there is still a height difference to the floor. REL is very specific about their subs (especially the ones with bottom passive) maintaining the correct height from the floor. Not sure if a platform is a direct substitute in that regard. 
I wound up making a small adjustment to the controls when I went from the platform to the SW feet.
Good luck with this. Hope the SW isolators will work out for you now.
I cannot say for sure if there was a lessening of my deep bass when adding the SW isolators. The reason is that my bass with the two REL footers was so bloated, overpowering and ill defined that I cannot make an accurate comparison.
My bass does seem to go rather low and has much more definition with the SW Isolators in my subjective opinion.
Had tried placing the Carbon Special subs on a pair of Auralex Subdude platforms. The lessening of the deeper bass was annoying. Much worse, the volume of the upper bass increased dramatically. So much so that there was no way to get continuity between the upper bass and the lower midrange.
Could not find a pic online of the bottom of the Carbon Special sub, but did find one of the REL S/812 sub so others will have an idea of what the two footers beneath a REL subwoofer look like.
Walt
I revisited my two Carbon Special subs and redid them following Walt1’s guidance.
(Before re-doing them I checked what my clearance was from the floor and it was 1 and 5/8 inch.)
After redoing them I was able to confirm that all the feet were now the required 2 inch clearance of the floor and the subs were pretty level, no rocking, so that showed me they are all pretty even.
This has allowed me to remove the Audiophile Base platforms I had both subs on.
I’ll get some listening done over the next couple of days and report back.
This is a review by Stereonet - along with some measurements - of these AUVA SW Isolators:
https://www.stereonet.com/reviews/stack-audio-auva-sw-isolator-review
Walt
I was able to test out the new ‘placements’ for my SW footers last night.
Much better, I felt I had regained the bass levels I had with the original Rel feet.
With the music system, bass was impactful, basslines were detailed and easy to follow, pretty happy with them now.
I couldn’t test out the theatre side of my system as my 5 channel amp has gone back to the manufacturers for a service after twenty years of service with me.
In returning to the SW feet I have been in contact with Josh from Stack Audio and he volunteered the following info regarding replacement adaptors.
any Carbon users will receive replacement bolts once this comes to fruition, we are very thankful for the innovative solutions that people have utilized thus far.
I have built a database of Carbon users so that the switch up should be seamless.
He did say they have been experiencing difficulty in sourcing the longer adaptors, hence the delay, but excellent customer service from Stack Audio, as usual.
As previously reported, I have gotten excellent results with the Stack SW footers on my Carbon Limited and one of my Carbon Specials which each sit opposite of the listening position on the side walls of my 540sqft room, aimed (to the extent that it matters at bass frequencies) diagonally toward the rear of the room.
Tomorrow, three more sets of the Stack SW isolators are due to arrive for my two stacked Carbon Specials sitting in the front right corner of my room, eight for the lower sub and four for the upper unit. I’ll need help installing them, but I should have them up and running in a few days…