Ok...I just did a luxon assassin run, killing about 25.
In the first run, I ran SS and body blocked them so they all died at the exact same time.
As you can see, I got 6 drops, three being chocolate bunnies. (Note; IP and Suffering were not used...Those are used in my normal Luxon Farm build, because I kill them in groups of four at a time, and don't body block them so one will always run off. Suffering stops him from regening and IP kills him faster than SS.)
Just a minute later, I did a run with SV and Necrosis
As you can see, I got 21 drops, three being chocolate bunnies. So it would seem that Mass agro lowers drop rates of non-event items.
The drops were never touched, both pictures were in HM and within minutes of eachother. The only difference was one used SS, and one used SV/Necrosis.
Oh, and manitoba1073, my test is better in that you only kill three, while I kill 25, and you can clearly see the difference. And I also can repeat my results every time.
MLiC