I just plugged an HD580 into my GV6/8620 and to be honest I was not that delighted by it. Superficially it sounds fine - quite nice, even - but there's a lack of transparency to the sound, as if there were sort of a light scrim lying over portions of the music. Things that should be sharp and clear are sort of spread out and diffused, and sonic grain and texture are softened and slightly burred. Perhaps this is the fabled Sennheiser Veil we hear so much about. I've never noticed such a thing running an HD580 from a Gilmore Lite, but running it from this little amp, I think I may have encountered it at last.
In all fairness, if you are new to amps and to headphones at this level then it is likely that you'll still be pleased with the combination. It does sound good -- it's just that I've heard the HD580 sound a good bit better.
A really big thumbs-up on driving the ER-4S with the GV6, though. Very highly recommended.
I also have a GV5/6171 and the earlier GV6 with 8397, and I can give the HD580 a spin with those over the weekend and report back, if you're interested.
Edit: if you're going to contact Norm, you might ask if a GV6M+24V Elpac would drive an HD580 well. You'd still have the very nice GV6 + Ety combination for mobile use, and for less-mobile use I wouldn't be surprised at all if the GV6 drove the HD580 noticeably better on 24V wall power than on 9V battery power.