Yeah, for anything under 20mW even a cheap amp makes a lot of difference. The DT770 were unlistenable on my friend's 14mW Cowon U2 - with the PA2V2 they worked quite well together.
The DT990 are open, so no isolation at all. They're also very bass heavy - at least the older version I tried. I heard they "tamed" the newer revisions a bit. As far as open cans go, the HD600 sounded more detailed, precise and balanced, also the soundstage was better. In order of personal preference: HD600 > DT990 > K701.