I used to use a 570/Marshall EL34 100/100 rack combo, back when I had a Rivera Knucklehead. (Edit: That would be 2005, actually :) ) In the end I sold both the rack and the Rivera and purchased a Splawn Quick Rod.
Now then, about the sound.
The ENGL is a fantastic high gain preamp somewhat in the Marshall vein; it has a very open top end and a really tight midrange -- all the way from the lower midrange to the upper midrange.
If you want to hear what my Engl sounded like, you can check out this clip on my band's web site:
http://www.twilight-odyssey.com/music.html. All of the harmony guitars are an ESP M1 Tiger going into the ENGL/Marshall rack.
My personal opinion is that the ENGL is perfect for power metal and euro metal, not much else. It's very tight, and is pretty versatile, but ultimately is a sound I equate with DragonFarce, Helloween, Rhapsody (both with and without the 'Of Fire' bit PDT_008), Circus Maximus, Edguy, Blind Guardian, Accept, etc etc
Ultimately there was a nasal quality about the ENGL that I never took to.