a lot. basically all the internal. like internal damping. internal acoustic film. internal acoustic foam.
driver position.
all i had tried.
getting one of them modified is easy. but getting all altered. and wanna get a good combination is PITA.
i will seriously advice you don't ever mess with the driver.
i have no turning back as i already alter it and found out.. "shhit.. this sounds really sucks compare to an stock AKG701."
and i consider i was lucky to get a great combination of what i alter. material i replaced. to work in a good sound with accurate tonality.
what sep90 wants me to do is.. "maintain the treble and mid of AKG. maintain the soundstage.. and reduce sibilance and increase in bass"

what i tested when recable it with pure copper cable. is the sound stage shrink. although get much more impact of bass. BUT the bass isn't hit deep
it's internal damping makes it sounds soft.
so i took off the internal damping. and it sounds like shit. even much more hollow than the original K701 (i felt the original 701 sounds very hollow especially in mid.)
i put it back again, try different position..
add some more damping at behind of driver... YADA YADA..
including of recabling process. i had opened up that specific K701 for more than 9 times.
and.. honestly. it's a nightmare.. each earcup has 7 screw.. with numbers of hatches and plastic hooks. lol

haha i glad u know my preference well since u heard my sened and u know how my hd650 i wanted it to be.....transparency, no veil , no more bloated bass and deep extension on highs and lows with exceptionally superb mids... 
congratulation on modding my akg for me ! great job there pcworks !! 
now i need to come back espically for my akg .......and do throughout comparison with stock 701 and APS's  701 recabled akg.....

what is APS?
a person?

hehe
some of great poison here....guys beware

spare part kot??

hehe u dun wan to know wat i do with my spare powercords......
hmm let me c  silver reference blacksand with canare f-12 will do wonders to my portable rigs or not.....

Lol bro, today you already make us crazy about akg701 damn nice with pairing my class A

lol... one and only. i seriously phobia to touch on AKG again.
=X

the neutral and transparency is the nighmare for me to mod and tune. it not only involve recable. but all the driver positioning, spacing, acoustic chamber tuning...
sigh........
so no more AKG 501,601,701 for me.

nope.
dun wanna touch it again. and i seriously meant it.

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Added on April 21, 2009, 1:55 amthe time i spend on it, is just too much. on fine tuning the sound.
with risk that i might kill the driver anytime as the driver can't be place nicely in anythings except it's own enclosure. i have to told my housemate not to even come near my table. and for 3 days i was like maniac guarding my table. lol. if they carelessly knock the table. and i was f***ed up.... some more in malaysia. how much i could charge for that? any price beyond RM 300 might be scolded by ppl. while i work full time for full 3 day tweaking ur 701.