Thanks for measuring these, Purrin.
As I contacted Purrin via PM about measuring these I informed him about my objectives with this project, which were as follows:
1) Answering the question if a closed Grado could work (even more so because it seemed common knowledge it was not possible)
2) Improving the bass extension.
3) Reducing or eliminating the Grado shout.
I ended up with a setup that sounded acceptable to my ears and I thought measurements would certainly tell me if I was on the right track or not. Listining to the headphones I thought I was, but you never know before you actually see the graph.
As for the 9k peak, I did not find it that annoying, maybe because I focussed too much on the bass and midrange.
Used drivers: Grado SR80i
Cups: Wood cups RS1 style
Bass vent: 8mm bass vent at the bottom of the cup (damped internally)
Damping: felt, medium density foam and sheep's wool
Driver damping: Now this is the tricky part. Compared to the Koss driver the Grado driver is a bitch to get working in such a setup. Whereas you just put a Koss driver into the cup and you have a working headphone, you can't just do that with a Grado driver. You literally have to choke it to get it working.
Now if I were Grado, I would use some shiny exotic wood cups, some drivers that have a better response in the higher frequencies, copy the StratoKOSSter pads and apply the StratoGrado damping scheme. Then I would call it the "Grado Revolution" and sell it for USD 1K. And make some dough.
@purrin: If you happen to have time, I would be very thankful if you could try the following and hear/see for yourself if the results changed for the better or worse:
a) putting some foam (like the Koss earpad foam disk) between driver and ear
b) lifting the drivers a bit from inside the cup to increase cup volume
c) changing the damping on the driver back (different variations of felt/paper)