So why did I create this thread? I recently mustered the guts to open up my HD438s to do some modifying, so yes I'm fairly new to modifying. I suppose this guide could help other newbies too. The methods in this thread have been covered in several other threads, but there was no thread dedicated to the modding of Sennheiser HD4*8 models so I thought I'd share.

The internals of the HD418, HD428, HD438 and HD448 are very similar, so I believe these mods will apply to all four headphones. Be extra careful with the wires and the drivers. You do not want to damage anything since opening your headphones up will void the warranty.
Legend: Blue title = sound mod
            Brown title = cosmetic mod
First off we have the,
Tape Mod 

This is what it looks like. [Disregard the 4 holes, that's for later] 
Step 1: Detach the ear pads from your headphones. This requires quite a bit of force.
Step 2: Get some double-sided tape and tape the sides of the circle ring as shown in the picture.
Step 3: Attach the ear pads back on, all you have to do is clip them to the tiny hooks on the edge of the oval. 
Step 4: Press the gauze of the ear pads inwards towards the drivers to ensure the gauze sticks on to the tape.
What this modification does is it reduces the vibrations caused by the gauze of your ear pads when playing music at a high volume. Personally I don't think this mod is that important (I did it anyway) but if the gauze vibrations bother you, try it. 

♞ There should be no noticeable differences in sound quality.                                                                                 

The next mod requires you to open up your headphones even further. What it does is increase/decrease/tighten the bass on your headphones, depending on your preference. By default, there is a black mesh that covers three holes (iirc). This image below shows the backside of your HD4x8 headphones' driver enclosure.
Bass Mod 



Step 1: Use your fingers to locate the 4 screws hidden under the grey foam. 
Step 2: Get a Phillips screw driver and twist the screw out from above the foam.
Step 3: Once all the 4 screws are out, pull the driver enclosure out by pivoting from one end of the ear pad hooks. 
Step 4: Remove the black mesh. 
There are a couple of configurations.
Leaving all the holes (which were initially covered by the black mesh) uncovered will give you a full-blown bass. There will be lots of bass but the quality of the bass will not be as refined as it was. I suggest you experiment with the covering the holes with tape (this involves repeating steps 2-4 a couple of times) to get the best results. 
You can also try adding some deadening material in the chamber that surrounds the driver enclosure to tighten the bass, dynamat or cheaper alternatives would be good for this.
♞ There is a huge difference in sound before and after this mod. 
♞ Left and right cups have different internal wirings. 

Dubstep mod (koolkat) 
  • Remove all the tape covering the 3 holes. Leave them uncovered.
  • Spread out blue tack on the insides of the cup or on the back of the driver housing (refer to pics)
That's it. 
By right, leaving the three holes open muddies up the sound due to the massive increase in bass but the blue tack helps to dampens the resonance in the chamber. The result is real heavy bass impact-wise (good for dubstep and similar genres) but it still has that edge in bass guitar. Without the blue tack, guitar sounds really blunt.
It's a reversible mod, so give it a go.  

Resistor mod (Credit's to Solderdude at RockGrotto) 
You can also do the resistor filter mod for the 438 that makes the frequency on the low's more prominent and revealing by soldering a resistor to filter the different frequencies on the low end. The process can be done by following Solderdude's first post with the pdf file.

HD438 cups without the chrome spokes

Modifying tips:
☛ Place all the screws in a cup.
☛ Preferably do it on a table with a large surface area with ample lighting.
☛ Careful with wires, you might accidentally rip them loose.
sbwf mod
bass mod
Resistor mod