I have the 449 and I wanted to share this. If you see the inside of the 400 series, the hole basically function like a speaker bass port which I believe you guys are trying to mod it. If you guys have experience with speakers, basically speakers design with rear bass port are very sensitive to speaker positioning and need a good solid wall to reflect bass. Floor standing might have bass port closer to the floor in addition to rear bass port design. Of course there is front ported design which need space very much like open back headphone. Bookshelves speakers also benefits a lot mounting on a heavy solid stands, you get tighter music, better definition and separation kind of thing. If you place a speakers on a hollow bookshelves or table you started to hear boomy bass or defuse sound if you know what I mean.
Back to 400 series, inside the headphones, these holes are basically bass port. It deflect bass and because the outer shell of the 400 is actually quite thin. The bass produced is more or less not to it's optimum performance. If you play music and if you feel the outer shelve the vibration is quite hugh. Basically the back of headphones does store energy and different material such as plastic, wood or metal will affect the sound.
Tyll explain this on the HD25-1 II vs Amperior:
Based on my many years of experience with speaker systems, my objective to improve the sound of 449 is simple. Improve the material of the 449 outer shelves so it is firmer, less resonant and give more focus in sound especially treble as well as better definition and impact to the bass.
I bought some black tac (black version of blue tac, black one is softer and more stretchy too) which I use to improve my Grado. I will talk about it separately in Grado thread. I wanted to open up the inner capsule but failed to do so. So I started of to reinforce the outer shelves to metal.
I bought a sheet of aluminum foil from Art & Craft store and cut out the shape (some trimming needed) so it is the same shape inside the outer shelves behind the bass port. I spread a very thin layer of black tac adhesive material to hold it in place. The black tac in between also act as insulation material to absorb vibration and resonant. Now by tapping the back of the headphones, it is no longer hollow sound. It is much solid than the bare shelve.
I also realized the space between the inner capsule and the outer shelves is very small, since I failed to open the capsule (I still think if I can spread a thin layer of back tac on the inner capsule wall, add a thin layer behind the metal magnet-tricks grado did to RS2 and higher) I rolled some black tac on the outer capsule to it will stick on to the outer shelves. You don't have to put too much just strategically and please do not block the bass port. What this does is that the capsule is not more or less one piece integrated to the while headphones and with the black tac active as insulating material against vibration the sound will be very focus. Somehow the 449 driver has it's potential, I feel the separation is actually very good and the sound signature is very clear.
The mod will give you better, tigher bass. The treble will also improve quite a bit. It no longer feel like a good speakers position wrongly or sitting in a hollow bookshelves (thin, hollow plastic back).
What do I benchmark with? The HD650, MS2i and SRH840 are some of the best headphones I have and before the mod, these are clearly better than the 449. After the mod, I have achieve what I initially plan for, to bring out the potential of the 449 and I can now confidently said the comparing with the SRH840 and MS2i the 449 feel it is on the same league although each of them has a different sound signature which there is no way to say which is better (Although I would say HD650->MS2i->SRH840|HD449).
Previously I always feel that the 449 while good is a lower league model than the MS2i/SRH840. The bass while clear seems missing that little something, treble might be a little too dull and the mid is clear but de-focus. It is the same way like I said base on my experience with speakers without a proper stands or position at the wrong way.
Try it, watch Tyll video above and see what I am trying to say. Too bad I didn't take any photos especially the metal disc I made to reinforce the back.