Ok so I've put in 50+ hours of burn in on both of my HD 439s running random music and 'pink noise' through them. I tell you, the differences between my unmodified set and my modified set is quite large and they're like two completely different headphones now.
Both sets have the pad filters removed. I believe these are there primarily to block out hair, dander, dirt, etc from entering the driver. However they also muffle the sound by a good margin (I even tried talking through one of the filters and the projection of my voice was noticeably reduced). Getting rid of the filters help to distinguish better detail in pretty much the whole audio frequency (including vocals) and improves on the soundstage.
The HD 439 is marketed to have enhanced bass. However, I notice it doesn't produce the level of sub-bass that you would expect from a headphone marketed as such. After burning them in, the tinnyness is just about gone. What's left is a nice set of cans that plays smooth, detailed music with clear vocals and with a good soundstage. This set is a bit on the cold side and produces laid back mids and highs for its sound signature. Actually, the whole sound signature is laid back. It has decent upper bass (thumps) but completely lacks on any kind of low-end bass (rumble). The kind of bass that you get from a headset right out of the box.
Modified -- bass mod and blue tack mod:
The biggest improvement from these mods made was to the bass so I'll touch that first. The bass has really opened up. There's a lot more rumble you both hear and feel. It is a healthy rumble that shows on parts of songs that did not exist before. Opening up the middle bass hole made the modded Sennheisers have the ability to extend into the low frequencies and brighten them up but also seems to do a good job controlling it. I don't notice bass bleed but although still tight the bass has loosened up. It finally has a level of bass most rock and rave fans require.
The mids and highs has also improved. As a matter of fact, they are more pronounced and textured. It's not often that you can mod a headset to have a lot more bass and not have it hurt the mids and highs. The vocalist is now right in front and so is with the guitars and drums. It feels that it's less airy now that the sound spectrum has improved so I believe this results in a smaller soundstage but ambient instruments still remain ambient and you can tell they're in the distance. Moving forward, the headset's ability to reproduce detail has slightly increased and the clarity doesn't seem to be affected. Nothing I notice has been muddied or become harsh. Any more color would have made songs too forward and not as pleasant to listen to.
In summary, the mods has changed the HD 439's sound signature DRASTICALLY and I'm sure the same applies to the HD 428s or any other 400 series Sennheiser. It now plays songs with a lot more energy and with better detail however the characteristics of an airy soundstage has suffered somewhat. Still, it's so much more fun to listen to that I will be modding my other set as well and use it as my second. I don't have any experience with other cans but I'm sure it now measures up to sets that cost a lot more, and I wholeheartedly will recommend these to anyone looking for a quality all-around headset without breaking the bank.