I use the Sennheiser HDVD 800 with the HD 800's and I'm really happy with it. I just installed Audirvana + and play hi-res on MacBook Air. Control iTunes with Apple Remote app on iPad. One word - Awesome!! I can't say I auditioned a lot of headphone amps/dacs though. I ordered the amp through Amazon and figured I could return it if I wasn't happy. But I'm happy to say that wasn't necessary. Let us know what you decide!
Damien,
just installed Audirvana + and have to say I'm blown away by how great it sounds. Amarra has been a hassle, with a clunky UI, and I'm so happy to have found the software on this forum. :-)