Here is to be collected links to the most instructive posts on how to modify the Fostex T50RP, mostly from this thread: Just listened to some Fostex T50RPs today... WOW!
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow
 
This is not for "Thunderpants" - only modifications keeping the standard cups
 
 
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BMF:
 
BMF is bluemonkeyflyer's mods.
 
BMF's Fostex T50RP Incremental Mods and Measurements (FIMM): FR and CSD/Waterfall Graphs for Different Configurations Including DBV #3
http://www.head-fi.org/t/618659/fostex-t50rp-incremental-mods-and-measurements Includes:
1. DBV #1, DBV #2 ("Dmitry's Mod"), and DBV #3 ---> DBV #3 is recommended 
2. Tutorial and Pictorial for "Simplified and Consolidated Tutorial and Pictorial for DBV #3"
3. DBV #3 Variations with Grodan Rock Wool Mod, "Naked Driver" Mod, and Rite Aid "First Aid" Cotton Balls Mod
4. Tuning Rules of Thumb for tweaking your "Initial Build" to best meet your preferred sound qualities and "audio chain"
5. "How To" make, implement, and test Modified Bass Ports (Temporary External and Permanent Internal versions) and Modified Baffle Ports
6. Repairing Stripped Cup Threads and Hanger Holder Threads
7. Pad Rolling Measurements for 8 Pads using an All Stock T20RP mk2
8. Hundreds of Incremental Mods and Measurements "How To" Pictures and measurement graphs (FR, Waterfall, Spectrogram)
9. Preparing Rite Aid "Absorbent Rolled Cotton" and J&M Fiberglass
10. Using templates and a digital scale for balanced Left and Right Channels
11. Comparison pictures and "All Stock" measurement graphs of T50RP, T40RP mk2, and T20RP mk2
12. Comparison measurement graphs with Panasonic WM-61A mics mounted on Etymotic Triflange Ear Tips vs Etymotic Foam Ear Tips
13. Comparison measurement graphs of my LCD2 v.1 headphones
14. Recommended installation of Dynamat on the ear side of the drivers
 
BMF 8.1
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/8025#post_8354423
This link includes:
1. V8.1 Mod Tutorial Text (Outdated - Use DBV #3, above)
2. V8.1 Slide Show (Outdated - Use DBV #3, above)
3. Room Equalization Wizard (REW) Setup Summary
4. Links to BMFs Incremental Mods and Measurements - REW .mdat files - Useful for learning how to use REW
5. Phantom Power Supply & Panasonic WM-61A Mic Slide Show - "How To" Build Your Own Measurement Kit
6. A Relatively Simple Mod Variation (Outdated - Use DBV #3, above)
 
All the following mods are Outdated:
BMF 6.3
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4290#post_7856037
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4440#post_7865771
BMF 6.1 vs 6.2
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4155#post_7840197
BMF 6.2
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4125#post_7837614
BMF 6
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/3945#post_7827522
BMF 5
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/3840#post_7823213
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/2640#post_7768186
BMF Transpore mod
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4875#post_7900702
bluemonkeyflyer's modified rastapants with picture walk through 
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/2655#post_7768646
more from bluemonkeyflyer 
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4620#post_7877085
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4560#post_7872751
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4365#post_7861181
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4245#post_7851889
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4035#post_7832914 (applying dynamat) 
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/3045#post_7790852
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/1785#post_7582437
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/2715#post_7771999
bluemonkeyflyer on temporary closing the cups in between mods:
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/5340#post_7949196
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/5295#post_7945865
bluemonkeyflyer compares unmodded and modded T50RP to LCD-2 etc.
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/3690#post_7816428
follow up
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/3735#post_7817267
simple mod
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4440#post_7865771
 
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RASTAPANTS:
 
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/6135#post_8119267 Rastapants 2 by mrspeakers:
Rastapants 2 840 Pad Edition:
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4260#post_7852863
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/3525#post_7808672
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/3525#post_7808673
 
Rastapants 1 by mrspeakers
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/2445#post_7746934
Rastapants by mrspeakers pictures
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/1380#post_7357008
 
mr speakers measurements
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4200#post_7846094
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4185#post_7845429
 
mr.speakers sbour RP2 and O2 pads
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/3705#post_7816511
 
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HDman T50ResPonse
http://www.head-fi.org/t/583371/t50-response
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/5220#post_7938002
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/5175#post_7932721
 
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OTHER MODS
 
Arleus on modification and measurement - Spectrograms and SPLs
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/8085#post_8362915
 
mmda - simple glue and clay mod
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/6060#post_8109855
 
BmWr75 simple mods (a BMF variant)
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/5745#post_8037638
 
LFF "Paradox"
It is a secret.. elaborated here
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/5295#post_7947203
 
Magick Man's "Smooth Operators"
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/6075#post_8114313
 
hans030390's modded T50RP (Updated 12-15-2011)
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/5490#post_7972850
 
Fat Bastard T50RP by snakeeater69
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4995#post_7911193
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4485#post_7867736
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4470#post_7867493
 
Very very easy mod with K240 Sextett pads by RexAeterna
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4965#post_7908749
 
mmayer167 easy mod with O2 pads
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4635#post_7877318
 
rawster picture walk through RP2
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/3885#post_7824377
 
SPwild's mod 
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/330#post_6998354
 
Cheap mod by dogears
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/2445#post_7748868
 
Jonclarke doing tidy work with Paxmate and Dynamat - pics
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/1575#post_7436860
 
PhaedraCorruption picture tutorial
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4485#post_7867505
 
ambchang's two mods comparison
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/5130#post_7926639
 
Faust mod:
http://wiki.faust3d.com/wiki/index.php?title=Image:T50RP_m2.jpg
 
b0wl1ngs variant:
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/1785#post_7583476
SlaRe comments: "Tried this one, seems like the best best if you don't have paxmate. It's impressive what just a thin piece of felt glued over the driver can do, really good isolation too."
 
Smeggy's old setup:
http://www.head-fi.org/t/111193/orthodynamic-roundup/6555#post_5220171
 
Rastapants in the flesh:
http://cdn.head-fi.org/a/a3/a33b16c8_2970acf8_rastapants.jpeg
SlaRe comments: "Works best with 840 or O2 pads apparently. The leeperry mod is basically the same thing, only you leave the white felt over the driver."
 
LFF headband suspension
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/2895#post_7782196
 
Twinster's sound reflection cardboard cookie
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/5490#post_7972312
 
Pwnm30rdi3's Cup Re-paint Tutorial:
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/8880#post_8549362
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/8880#post_8549419
 
joelpearce's Rock Wool Mod for "Naked" Drivers:
http://www.head-fi.org/t/571549/fostex-t50rp-modification-summary-links/30#post_7778717
 
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GENERAL ARTICLES
 
How to open and re-assemble while trying out different mods; How to remove and re-assemble the headband hanger
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/2640#post_7768186
Scroll down to "How to Open and Re-Assemble Fostex T50RP Headphones[size=11.818181991577148px]"[/size]
 
AnotherN00b sum's up modding
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/5175#post_7931332
 
Fostex T50RP Modification:  A Summary of Methods, Materials, and Their Possible Effects (Out Dated - No Longer Supported --> Go to: BMF's Fostex T50RP Incremental Mods and Measurements (FIMM) at the top of this page)
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/2640#post_7768186
 
Armageis on damping
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/3795#post_7820049
 
High End Phones-Comparison
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4230#post_7850268
 
Exploded parts view
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/1845#post_7610095
 
Tyll Hertsens explains about orthos in general and why they are especially sensitive to good acoustic damping
http://www.innerfidelity.com/content/how-planar-magnetic-headphones-work
 
Making and using a sound testing kit with mics, phantom power box, and Room Equilization Wizard
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/8025#post_8354423
 
A basic modding demonstration and explanation for those wanting visual guides.
http://www.youtube.com/watch?v=rgccbW6BXJY&hd=1
 
Megathread search tool by ludoo
http://headfi.qix.it/
 
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MATERIALS:
 
Paxmate
 
Acoustipack
 
Dynamat
 
Newplast
 
 
 
 
 
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PADS: 
 
Beware that pads have influence on sound, and the mods are adjusted to the pads. Always keep in mind what pads are used by a modder.  
 
Shure HPAEC840 Replacement Ear Cushions For SRH840 Headphones
Shure HPAEC940 Replacement Velour Ear Cushions for SRH940
Beyerdynamics DT990 Velour pads (black or grey)
Fischer FA-003 pads
Audez'e LCD-2 pads
Stax 007 pads  http://www.ebay.com/itm/390342410864?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649
 
Smeggy's description of various pads
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/4485#post_7868175
 
Useful "Sock Mod"
to avoid those platic earpads making you sweat. I did this - and ignored the cardboard part, just stuffed the cloth under the pads :)  http://www.head-fi.org/t/470304/shure-srh840-earpad-mod
 
 
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HEADBAND:
 
HD280 pad mod by satwilson
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/5505#post_7973404
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/5520#post_7980238 
 
Self-adhesive spare parts pad mod by ardilla
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/6045#post_8108975
 
 
LFF headband mod
 
micmacmo's leather suspension headband, inspired by LFF's headband mod
http://www.head-fi.org/t/452404/just-listened-to-some-fostex-t50rps-today-wow/8040#post_8356325
 
 
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Total makeover mods (new cups)
 
Thunderpants
 
KODA^2 / KODA^1 
http://www.head-fi.org/t/582736/koda-2-k-nigs-ortho-dynamic-acrylic-headphones#post_7927522
http://www.head-fi.org/t/582736/koda-1-the-ortho-dynamic-prototype-project#post_7927522
 
Leakypants