edit: I still have REW files saved from pure direct / no EQ in the signal chain, so I'll rerun the tests at some point so the comparison is done without any processing in the chain. The below comparison is with Audyssey and custom EQ enabled (but not changed before/after).
The Auralex SubDude and similar products are basically a foam mattress on a wooden board to decouple the speaker/subwoofer from the floor. Link to an example: https://www.auralex.com/product/subdude-ii/
I have wooden floors throughout the house, and things rattle and shake, and I figured why not - let's try it. It's extremely expensive for what it is - you can DIY this much cheaper.
I took some measurements before and after the installation at various SPL levels. There were no other changes. No EQ change, settings or otherwise.
See for yourself, it does appear to have quite a dramatic improvement at least in my current setup which is WIP. I've attached the REW files for anyone who wishes to take a closer look.
Some intense SPL gains which look odd, but are there.
Distortion dropped quite a bit above 30Hz.
Group delay seemed to improve as well.
The impulse response changed - I'm not sure if better or worse can be interpreted from this data.
Spectrogram without subdude:
Spectrogram with subdude:
And finally, the waterfall graphs. Without subdude: