Got my upgrade kit from Danny Yesterday, so I spent the afternoon upgrading my Rythmik L12 subwoofer.
A set of solid copper wire & a sheet of NoRez!
(I'll be getting to the tube connectors for my XLS this weekend after i get myself a crimping tool.)
Here's how the stock L12 looks inside:

Poly-fill sheets glued to the outer walls, & a single brace in the middle.
First step was to scrape off the poly-fill and any remnants/glue I can scrape off. Then cut the No-Rez sheets to fill every flat surface as best I can. It knocked down a large chunk of the cabinet resonance. I'm sure Danny's 3-brace design does an even better job than No-Rez alone, but this is pre assembled.

Next step was to disassemble the plate amp to get to the solder mounts for the driver and servo wires.
The hard part was removing enough solder & glue to pull out the stock wires so i could retin the thru-holes.
I made an error here I wasn't paying close enough attention, and wired the servo holes backwards. It
should be W-R-R-W but I did it as W-R-W-R. (I basically wired the servos backwards) not major, but it was an easy fix to just swap the wires on the terminal side once I realized my error.
Once that was done, I replaced the thermal paste that attaches the chip-amp heatsinks to the plate side of the amp. I have lots on hand thanks to my time building gaming PCs in the past.
Then I twisted each pairs of wires together, then twisted the twitsted pairs together & wrapped the wires back up with the small strips of polyfill & some zip ties. Next was to solder the wires to the leads & cover them in heat shrink. I marked the driver cables with permanent marker to know which is which.



Last order of business was to stuff the polyfill back into the sub, which I put over the plate amp, then seal it all back up! (Then find out i wired the servo backwards, forcing it into a nonstop loop until i re-wired it.. lol)
I haven't had a chance to play it with music yet, but it's definitely heavier and sounds a lot less hollow now. Should help reduce sounds radiating off the sides. And it was only $70 at most? Plus i have a little extra wire to use for my Tube Connector upgrade this weekend!

Enjoy!