After maybe a dozen or more hours I finally got my garbage T50RP to sound quite good. Not really all that impressive, but very good in some areas like vocals.
I thought this post might be helpful who couldn't get any good results no matter what they tried. This works for me and I doubt anyone has a similar configuration.
1) Add blu-tack to baffle
2) Remove felt under earpad (suggest trying the results first with it still on)
3) Add Akasa foam inside cup. Only 1 layer. I did this on EVERY single exposed piece of plastic. Don't use two layers. Right up to the edges!
This is where it got tricky for me. With the vent open with all this foam it ruined the sound completely. It made the sound extremely muffled and distant. This is before I did ANY other modifications past this point!!
4) Added a piece of felt to the back of the ENTIRE driver. Sounds crazy, I know! Micropore/Transpore does absolutely squat for me.
I tested this out first by adding one rectangle of Akasa foam right on top of the middle "post" in the cup. This made my ears extremely annoyed. I think it boosts the upper mids or lower treble too much...somehow. It's nearly touching that felt backed driver.
I removed this piece and it was slightly better and less fatiguing but still tired my ears out. I experimented by taping the outside vents with regular scotch tape. The sound was now a bit more clear and not quite as fatiguing. How does that happen?!
Right now I'm experimenting with material on the driver behind the earpad. Right now I'm using cabinet liner with holes in it and then a piece of very thin foam over that. You'd think the liner screws up the sound, but it has less of a negative effect on the sound than the foam! I'm using the default foam from my HD-600. The foam seems to help the bass in some way, however that's possible.
Sadly the above configuration is the only way I could make mine sound good
Previously no matter what I tried it was always so muffled and the highs were extremely chopped off! That's with all the configurations found in this and other threads. It wasn't overdampened and I'm sure of that.
I still find these headphones so fatiguing even when the volume is low, despite them being quite neutral sounding now.