This link for more specific guidelines for specific changes
http://www.head-fi.org/t/802704/th-...s-mid-soundstage-smooth-trebles#post_12447940
I have found some very interesting ways to tweak and alternate the sonic performance of the TH900 in a nice way. Very simple, easy, and fun. Even though just like many things on the changes of high quality gears, the changes are minor, nuances details, but is definitely there......and therefore, please, do not confuse other people.
I was able to bring about these followings in general
1/ wider, deeper soundstage
2/ less shouty vocal
3/ toned down trebles spikes famous in the TH900
4/ deeper sub-bass more slam to mid-bass and bass
5/ better linearity in the lower mid spectrum, even more details in the lower mid, and also bring mid more forward
First things first. It involves 3 different mods: Pad mods (irreversible), foam discs placement, and cup chambers dampening.
Pad mods: trim around the pads to expose these lock holes more opened. Take out the pads, but leave it on the plastic plate, and then trim around but by bit using a good scissors......do it slow....Do Not Overtrim it...you will render the pads useless.
*** Basically, the pads mod will allow your TH-900 to "breathes" the way it supposed to be, and it brings about the improvements of more detailed lower-mid, and deeper bass with bigger bass slam and most importantly, the linearity of the whole mid spectrum will improve, which brings about more soundstage and the 3rd dimensional***
*****Do Not Trim Your Lawton Pads*****!!!
Foam discs placement mods: there are 4 positions that I found to have been useful. It is found upon the 4 holes these foams were cut to slot in and lay on the screws guides. One will line up, the rest will be aligned automatically. Remember to remove and install the pad correctly to avoid the foam disc being moved.
1/ Stock: for stock sound , and of course
2/ stock Plus: yeah, it improves bass and more clarity and trebles details upon Stock sound....sibilants may/will present....depends on how much love you have for sparkling bright and trebles. Count these white holes away from the lock port , one, and two, align the foam hole on top of number 2 count
3/ #2 : most balanced and opened sound signature with strong slam and powerful rumbles....it is my favorite. Line up the foam hole on at the largest lock hole opening
4/ #3: the ultimately opened position for 900, brings about the ultimate bass rumbles, very deep deep bass that you probably never felt before, much better soundstage, but some lower-mid may be a bit recessed. Count the last hole at the end of the locking port mechanism, and line up the foam hole on top of it.

Video tutorial
[VIDEO]
Cups damping mods. To come next...but if you already have Lawton first stage, that will work as well.
[COLOR=FF00AA]Part II[/COLOR]
Updated cups dampening mod. It will further help to mainly control the trebles spikes, sibilant, and allow deeper sub-bass + slamming bass body
the cups dampening
Now! Your next problems (may be):
trebles spikes , and sibilants ?
1/ open up the cups, by unscrewing these red screws, and leave the green screws alone
2/ remove these white baffles stuffing inside the cup chambers
3/
then to [COLOR=FF00AA]preserve[/COLOR] the bass "rumbles, the body slam" and the airy and linearity of the mid spectrum nuances details + control the trebles spikes. We will use this tape. They can be found at "first aid" section in any stores, Target, Walmart...etc
4/ You will then tape up the cups chambers with 3 layers worth like this, or you can do a better job

. Now, if you hear "distortions", or you have the "patience" to do it, you dampen the drivers back like this picture (recommended, but not a necessity).
Only tape on the Black plastic, and Never, Ever tape over the White paper felt!! Or Gaps
5/ Now, if after these above and you think that your headphones still has the harshness and spikes in the upper-mid and makes the vocal appear shouty ? You can put back the stock batting filter but in this specific position, make sure that you will leave out this "green hole" and that the batting block is inside the "red-square". Simply put, you turn the filter block around and let the green hole "breathe-free"
Doing this will smooth down the "lower-Trebles" and "upper-mid" even further while only effecting the soundstage only a bit....however sound leak-In will be further isolated and may create the illusion of constricted soundstage, but Width-Depth are still the same

. this is it! The final step to Ultimately Dial tune into your own preferences on your beloved Fostex and is the Complete guide
Then put it back together and Enjoy! The final product! Banana the Papaya and silver cables are an exclusitivity
Finally, if you enjoy these mods, please report back and Rep up this button bellow this post.
Thank you