Ultimately, I think it comes down to personal preference and your home situation. I don't doubt that REL is correct about isolation. I actually posted this question to REL support before I was going to purchase the S/812, and found out that it has a down firing driver. I live in an apartment building and was concerned about vibration and noise transmitting downward. So I asked about using something like the Auralex SubDude and this was the reply I got:
"We recommend placing the subwoofer directly on the floor and allowing it to couple to the floor. The feet on the subwoofer are specially designed to place the down-firing driver at a specific distance from the floor to achieve optimal effect, and altering this distance by placing the subwoofer on an isolation platform can have a detrimental impact on the performance of the passive radiator. That being said, the front-firing active driver produces far greater output than the down-firing passive radiator, and in our experience if your main loudspeakers are not disturbing your downstairs neighbors then the addition of a carefully setup and tuned subwoofer should not add any extra disturbance."
I ended up purchasing the S/812 and the Auralex SubDude anyway, and have no complain about the performance. I rather have a peace of mind and enjoy my music and movie then having to constantly worry about whether I am disturbing my downstairs neighbor. Also not everyone enjoys earth shaking bass, and some prefer isolation to reduce floor rattling. Just read some of the comments on Amazon on Auralex, and see how many people are happy using one.
Anyway, that's my 2 cents.