Okay so i finally got around to putting the plumbers putty in, and the result? It is heavier... the headphone that is, not the bass. Audibly the difference was, well, if you paid really really really really close attention you might find a tad bit more of a punch, or a longer decay of the bass note, but seriously im saying a tad bit. Ill define tad bit as 0.01 Db. It isn't worth it, i had to fill the entire cup full of it to get to that point(and i did try it with different amounts) and it weighed twice as much as it did before. For a placebo amount of bass.
But cotton did lower the bass enough for me to think there was more bass because i had forgotten what cup i put it in. The cup that didn't have the cotton ended up being the one with more bass. Someone might want to try it in the 840, see if it doesn't lower the mid bass hump. I filled the cup with a very tight layer of cotton from a pillow.