I always thought the E9 was a good match for the HD-650 despite what people say without actually trying it. I even prefer it over the Asgard, but maybe not with the HD-600. It seems the E9+E7 would be a perfect match for any headphones that's considered dark or close to it. IMO it's better with Sennheisers than say a K701 or DT-880 maybe.