Just got my m100's a few hours ago and I've already found out a quick, easy and totally reversible way to tame the bass just a tiny bit,
I've just placed 1 layer of 3m transpore tape on the inside of the shield covering the v-ports.
the tape can be purchased here: http://www.amazon.com/3M-Transpore-Tape-10-Box/dp/B000K3HSIO
or you might be able to find it at a pharmacy for less.
The impact on sound was small but I think it's just the right amount to give just a bit more clarity and less "boom"
This might also be a good way to cancel out the slightly boosted bass gained when switching to the XL pads.
I also imagine you could add more than one layer to have a more dramatic effect.
Here are a few pictures of the tape applied