Wow, just tried this on my PC350. Amazing what an effect a small hole has... Using some HD650s as reference so it's not just in my mind. The PC350 started out sounding very anemic and bass light, but crisp and clear. Now they are more bass heavy than the HD650s. A touch too much actually as there is a bit of a hall effect now. I got a bit carried away and put 4 small holes in each side. I've covered two up and added some cotton wool, but it's still a touch too much comparing back and forth.
One thing I noticed in comparison to the open back HD650 is that the sound changes quite markedly as you move the speaker towards and away from your ear. Without the mod, I could press the cans into my head quite hard and get an acceptable level of bass. This was at the point of my earlobe hitting the grill though. Pull them away slightly and they immediately sound tinny. The HD650 don't change in sound signature nearly as much, mostly it is just the volume that changes as you move them away or towards the ear. This implies that tuning a sealed can is much more tricky and will change based on head and ear shape as well as the condition of the pads.
Some tips:
1. There are 2 screws hidden under the foam sticker on the left side. Took me a while to find the second one.
2. Be careful with the wire that exits the left speaker at the bottom. I put some stress on this trying to find the above screw and once I did open it found that the wires had separated from the sleeve inside.
3.
There's a tiny rubber pad at the top of each driver that softens the impact against the headband frame. This is held in place by the driver assembly and so is free to fall out as soon as you pull the driver away. Find this and put it to one side before you lose it. If your driver assembly does not go back in easily, you might find it has fallen out and is in the way of the driver assembly
One final thing - there is what looks like a filter on the PCB for each driver (some resistors and capacitors). You can see a separate circuit for the mic and then sections for each of the left and right drivers. I'm sure this has some effect on the sound vs what is in the other headphones that use the same driver. This is also something to consider when re-cabling.
Still listening and I do have too much bass now...
4. make one small hole at a time and then listen for a bit, it's easier to not make the hole than to try and seal it up again
After another couple of days...
5. Too much pressure on the inside of the outer cup will make the middle part (the piece with Sennheiser written on it) separate over time. It is glued on with a type of glue that remains sticky and can be pulled apart. So, don't use too much wadding back there, or allow that piece of rubber I mentioned in 3. to fall into the back of the cup and put pressure on it. Unless you want to go for an open headphone design