Look closely at where the Pads connect to the cups. You'll notice tiny little tabs that go all the way around the cups.
Gently push the pads inward until you release a few of the tabs. Be careful not to snap one. (Look at the pic below and you can see the tabs around the housing)
Locate the 4 small screws that are under the felt lining:
Step 3: Gently remove the driver from the housing. Be gentle, move around the headband arms if the driver is stuck. Take your time as the inside wires aren't very long. You don't wanna pull too hard to separate the driver and housing.
Locate the Black tape covering the back of the driver:
on the 439 there are only 2 holes under the tape. The tape is thinner then the 428/9/448/449
So you will actually see the 2 holes. You can either just poke out one hole, or I use a razor blade and actually score around the hole and lift off the tape. Just do one hole, then put everything back together in reverse order. Try it out and see if it's enough Bass for you. If not remove the tape over the other hole. One hole should work fine. You'll notice the increase as soon as you put the phones on.
For different pads, just look at Sennheiser's website all the 400 series are swappable. Just get you some 428/9 448/9 fake leathers.