I see you have two subs. I have two sets and one will be out for at least a couple weeks. You might be best served by getting them from Stack direct as they should be available soon, and if they don’t work out you can return them under their policy.
I think the other Paul’s exuberance might be a near Axpona launch reality. I will be most impressed if they can pull off some working amp models with a future delivery date by then.
How do you deal with the height issue? REL advises against adding footers under their subs because their engineers determined the best distance from bottom of case to floor. Do you remove the existing feet from the REL and mount the footers to be the same height so as to maintain that same height from bottom of case to floor?
Carbon Specials have two runners either side of the cabinet underneath to allow for stacking.
I had an email from Stack Audio.
A Carbon Special owner removed the runners and Stack Audio were able to determine the thread size for Carbon Special owners so the Rel model should be specified at the time of purchase to allow for the correct bolt thread size for connecting the footers to the sub.
Does anyone know the price for a set of 4 footers?
Are they specified in different sizes like the speaker footers, i.e. the 50’s 70’s and 100’s?
That’s worth looking into. I have a Rel S812 6 pack, which is basically the poor cousin of the Carbons. The stacks were walking across my wood floor, so I solved that issue by lining the underside of the bottom runners with BluTack as recommended by REL. I do have some concerns that this could damage the floor’s finish over time, and it is impossible to do any fine tuning to the sub positions. If the Stack footers would solve this problem by providing enough friction to prevent the walk-about behavior while allowing for micro adjustments, that would be nice. Does anyone here have experience with using Stack footers with RELs in terms of SQ?
As per above, the Stack Audio SW footers are new and not even on their website yet.
They sent a few out to ‘beta’ testers, which is what we are seeing on this thread.
I’m pretty sure that the same SW footer thread will cover all Rel six pack subs so the same footer and thread will suit both S812 and Carbon Special subs.
Me being one of them, I guess. I’ve been so busy with other things (and not having gotten a helping hand from a friend) that I haven’t yet gotten them under my Carbon Special. Greatly embarrassed. 
Got an update from Josh today.
It’s the same size of footer for all subs but what they do is put a different level of CSA compound into the footer which allows for different weights.
I think it goes from CSA1 to CSA6, with 6 being for the heaviest subs..
For the Carbon special, which weighs 38kg, a CSA4 is required, which goes up to 40kg.
He also sent me a Newsletter that had a lot of beta tester reviews and they were all very positive, not only tightening up the bass but enabling the sub to better integrate with the rest of the sound.
The footers are being launched next week.
Hmm. This is troubling. When I did early adopter / beta test, Josh didn’t ask me what sub I was using. 
Perhaps you mentioned what your sub was and he
looked up its weight?
I did not.
Any real reviews from owners about the Stack Audio SW footers?
As per my post in a different thread, as soon as I get the right ones for my sub. A bit of a foul up on the first order received.
Would you put the feet under the REL base support? Or would you remove the support base of the carbon specials?
I checked, no room to add the footers alongside the existing runners.
They’re held on by bolts, I assume two each side.
So the runners need removed and Stack Audio will supply the correct bolts to connect the footers to the sub.
They are Allen wrenches. I have a full set in both English and metric, but nothing seems to be fitting
Perhaps it is a Torx or Torx Plus.
My BMW S1000RR incorporates many.
Josh told me that a beta tester was able to remove the Carbon Special bolts that hold the runners and Stack Audio will supply the bolts with the correct threads.
He said Rel use the same bolts for all their main subs.
Shout out to everyone that the Stack Audio US fulfillment center is up and running. Please see their website for more details and to get info on the new SW AUVA footers.
As per communication with Josh from Stack, the runners are removed, and the footers are attached using the REL specific threads. In the case of my S812 six pack, since the weight exceeds what a set of 4 footers can take, 2 sets are used. One set threaded into the mounting holes for the runners, and an additional set with the flat plates, with height adjustment done to create even weight across all 8 footers. I would like to hear feedback about SQ from any who do this setup under a six pack.