I took apart and cleaned out my other mag 350 today after being in service 5 months as flow was finally starting to slow. As usual although the outer circumference 120 square inch blue-bond can be rinsed and reused and inner circumference polyester polishing battens get so inundated with dirt that they become black and heavy and have to be discarded. I have never seen a polishing pad on my stacked canister Sunsun or Fluval 404 ever get anywhere near this dirty, mostly becasue if they could they would stall and stop flow. The reason why the Magnum can get this dirty and still run is becasue of the substantially larger surface area. While white (now black) poly battens capture all the dirt, the outer blue-bond layer and especially the inner carbon basket house a substantial amount of nutro-bacteria. Mostly because of the carbon which compared to most other boi-media surfaces is substantially more supportive as one teaspoon has a bacteria support area of 57,000 SF or about 1.2 acres. The 120 SI circumference blue-bond & poly-polishing layers coupled with the high flow center bio-carbon basket make this filter a powerhouse, or as some say work-horse.
Oh well, it's never fun enough unless it's difficult to begin with anyway! 
I'm going to get this set up tonight, I think.