Korg DW8000, Mono/Poly, MS-20...

> is it possible to mod the EX8000 like a Poly 800 with for example
> cutoff- and resonance mods?

Sure - but why because you can control these parameters by Sysex.  I
control my DW8000 using two Phil Rees C16 Midi Slider boxes:

http://www.philrees.co.uk/products/sliders.htm

Although if you do want true analogue fiddling - pots and resistors to
the cut off and resonance pins of all 8 filters is the way to go. 
Then you can twiddly really quickly with no audible stepping.

However, the sysex way means you can record your twiddling in the
sequencer ;)

BTW: I Modded my Poly 800 here:
http://www.strellis.com/poly800.shtml

Cheers

Chris