dt770= bass king of the beyer's family - closed can...couldnt tahan listening for 3 mins  superbass heads will love this cans
dt880= semi closed/open cans- balanced,neutral not so bassy or boomy- very linear and precise -for ppl who love neutrality and more into soundstage
dt990= open cans - bass r very deep,mids recessed and highs r too shrilling for me to handle....great for bassheads into metals/rocks/fast paced musics

closed cans = headphones that has no sound leakage to outside which is extremely good if you don't really want others to hear what you're listening at that moment

Open cans = opposite to closed cans so but it provides better soundstage and clarity.

last improvement to my beyer.....
just changed the velour pads to lambskin leatherpads...
now it is a good looker and the sound hehe 

way better than velour pads....it can last for years if taken carefully
lambskins pads r used for stax headphones and mainly used for audiotechnica cans...
velour pads is comfortable but get very irriating during long wearing hours and get very dirty easily
found out the that velour pads sucks all my dynamic bass and no wonder it is very hard to drive ...the original pads r holding the bass back ...
with this ahaha the bass whack and slams all the way down while taming the beyer's infamous highs and sibliances

Hehe I really love my Dt770 pro 250s. Sounds good on every song I tried so far, even low bitrate mp3s sounded good.

hey , any idea what amp to drive DT990 250ohm ? budget is about RM500 ~600 ==

wah the dt770 pro 80ohm is post 2005 model

hahaha
some digging in my store and found out some bipolar ribbon driver given by tachikoma to me couple years ago. lol...
perhaps i shall mod this into my DT770. lol
make it unique..
haha

Yo reporting in with DT231, next target DT440? Those cans so seducing...

http://jaben.net/forums/index.php?topic=17042.0

NOOOOOOOOOOOOOooooooooooooooooooooooooooooooo sucks to be malaysian

*i will be in line 6am in the morning if jaben malaysia doing this insane bundle  *

There's a very interesting link on Beyer's website that should answer your question and more. Here's the link http://europe.beyerdynamic.com/service/faqs/kopfhoerer.html

dah la try la when i went to jaben to send my ad900 for warranty lol. initial objective: send headphone warranty ended up: auditioning headphones/amps for few hours. lol