Current Amumu reduces the MR of the targets of his autoattacks by 15/20/25 for 3 seconds. The Amumu showing up on PBE inflicts a debuff which inflicts an extra 15% of magic damage that champion takes as true damage. These are _basically_ functionally identical, I thought.
Then I did the math.
All champions begin with either 30 or 32.1 MR. Those who begin with 30 MR do not have it scale. Those who begin with 32.1 have it scale to 53.4 at level 18.
This is not actually representative of the game in actuality, as flat MR glyphs are more-or-less ubiquitous. At 9x1.34, this is 12.06 additional MR almost every champion begins the game with. Therefore, it can be said that the average champion will begin the game with 42.06 or 44.16 MR, with those who have scaling MR going to 65.46 at level 18.
Now would be an appropriate time for the introduction of the formula for MR's damage reduction.
At level 1, this means that every champion in the game is taking ~30% less magic damage. For those who scale, they take ~40% less magic damage at level 18.
Which brings us to our first testable. Amumu's old passive at level 1 versus his new passive.
42.06-15=27.06. Plug this into the MR formula, and that gives you ~21% reduced magic damage, for a total of a ~9% increase in magic damage.
Obviously, Mumu's new passive outstrips that by far.
For melee champion MR, the calculation is similar. Blah blah 44.16-15=29.16 and plugged into MR that's ~22.5% reduced magic damage, for a grand total of an 8% increase.
The conclusion speaks for itself, so it's off to the opposite end of the spectrum. At level 18, Amumu will long since have had his passive upgraded all the way, and melee champs will have all of their scaling MR.
42.06-25=17.06. Another run through the MR formula and we have a result of 14.6% reduced magic damage. That's a big fucking swing from the ~30% MR that that squishy had before, to the tune of 15.5% increased magic damage!
Uh. Wait. His passive increased magic damage against the squishiest possible targets at the maximum level to about 15.5%. 15.5-15=**.5**
Point. _Fucking._ Five.
With flat MR reduction, which works at its absolute best against targets with already low MR, like squishy ranged champions.
Fuck it, moving on.
Melee champs, level 18, yadda yadda.
65.48-25=40.48 Do the MR dance and that's 28.8% decreased magic damage. So about 11% increased damage. To the tankiest champs in the game. All before anybody has so much as even thought about buying MR.
You know, I was going to go through this whole spiel about MR items, and average purchases, and how MR as a stat has kind of gotten fucked anyways? Yeah, that was a thing I was going to do. But then some research and some basic goddamn math told me everything I needed to know, which is that these Amumu changes are MASSIVE FUCKING BUFFS.
And that's all from the MATH. Don't even get me started on his ult applying his passive or his W refreshing it.