Thanks. The sound difference is likely due to the DACs, not so much the amps. The lower power of the Grace is more than enough for the HD800s. I live in the boonies, so I'll not likely get the chance to audition the Grace. :-)
The Magni 2 Uber is considerably more powerful than the Magni 2. If your headphones benefit from more power, then the choice is easy.
You can check the specs of both here: http://schiit.com/products/magni-2
Note that the Brainwavz Angled Pads are available in both velour and pleather. I don't know what effects the pleather pads create, but I am pleased with the smooth response that I get with the velour. No lack of bass from my perspective and no excessive sibilance.
I found that the Brainwavz Velour Angled Memory Foam Pads work very well, and are very comfortable. It is a tight stretch, but they look custom once they are installed. I got mine from Amazon for $25/pair.
I am finding that my Pono and Philips Fidelio X2s are a lovely match. I am just running single ended at present. My usual volume setting runs 50-75%.
(NOTE: For me, the X2's needed a LOT of burn-in before they began to sound well balanced. I'd recommend 50-100 hours. Straight out of the box they sounded overly bassy and muffled.)