I have tried to enjoy the HD800s on the a-100. Just not working for me. They pick up on too much. Have had a couple of people try the combo and while their impressions aren't as negative, no one's come up with anything overwhelmingly positive.
Strangely enough the HD580s sound pretty well with them, so I'd imagine it's not a dynamic thing, just a HD800 thing.
In terms of safety - will usually keep it around 9 o'clock. I have an additional 600ohm adapter lying somewhere but have not tried them together. Others have documented their usage on this thread and seems like no one's blown anything up as well.