I am looking for the name of a composer/experimental musician who did
a piece where he compressed all the major symphonies
(bach,beethoven,mozart etc.) into a single soundscape... I was told
about the piece some time ago but never got the name or was able to
hear it so i'm not sure of the details...

I couldn't find the piece you're looking for, but I can suggest two
other musical compositions that you may find interesting.

"The Missing Symphony" is a compression of all of Shostakovich's
symphonies rolled into one:

http://brainwashed.com/vvm/releases_vvm_/vvmtcd10.htm

Here's a review of "The Missing Symphony":

http://www.noripcord.co.uk/reviews/V/vvmalbum.html

Similarly, Pierre Henry's "La Dixième Symphonie" combined many works
by one composer (in this case, Beethoven) into one piece:

"In 1979, at the BeethovenHalle in Bonn, Pierre Henry created his
'Tenth Symphony of Beethoven'. This work obtained 'the French
Victories of Music' in 1988.

Ten years after the prize, Pierre Henry created a remix of his tenth symphony.

The first hearing is without doubt not easy. A contemporary composer,
Pierre Henry researches the sounds, effects, sentiments... But to draw
out the ten movements, one after another, the harmony made and please
the listener to understand the composer's admiration for Ludwig van
Beethoven."

http://www.lvbeethoven.com/Cedes/TheCds_Others_Curiosities.html

that is probably my guy...
thanks a lot