Step 2: The mod
First off you have to open up your headphones.
I recommend starting at the side where the cord connects, this is the harder one and then you got that over with.
First remove the ear liner (pic #1)
It clicks off, so it takes a little bit of force to remove it.
Then remove all the screws on the earcup (pic #2).
There are two hidden screws (the lower ones), you have to stick the screwdriver through the bit of tape (this is the part that voids the warranty). One of those two screws is a different size, set it apart from the others.
When all screws are removed, carefully take out the driver and flip it around.
I say careful because there is a short wire that connects the driver to the cup, break it and you're in trouble. (see pic #3)
Now place the drill (see pic #4) and slowly drill ONE hole into the area indicated by the arrow.
Take care not to insert the drill to far or make too much revs. Bits will end up in with your driver which may ruin the sound.
Take you time on this, it's an expensive headphone, you want to keep it working.
After this is done, flip back the driver and put back all the screws. Do not tighten them all the way immediately, but insert all screws first, then fully tighten. Also, you can see (pic #5) I'm pushing on the cord connector. Otherwise it could be possible that you can't tighten the screw right, which might lead to breaking of the soldering points for the cord.
Lastly replace the ear liner. Just push it in place, it give a nice click when properly replaced!
Now rinse and repeat for the other side!
If you find that there could still be a bit more bass, you could widen up the hole a little bit, or add a second hole.
However, if you make them too big, you will end up with a lot of BOOM and no fidelity at all. Which was the reason I closed up the vent holes again.