Another point that I didn't see gain focus, and certainly should impact the buying decision for a powerfull Headphone amp is Gain.
If I recall correctly, the A90 has 19+ db of gain. While this one gets downgraded in that regard.
I own an SP400 and I struggle with the lowish gain as I eq my headphones.
If I listen to a michel jackson or stevie Wonder song, my headphones don't get loud enough even at 99 (for other songs it is not the case).
I feel it is a compomise due to the discret nature of the product. Topping reduced gain to keep Sinad high.
Too bad, as I'll have to keep unsuccessfully looking for an upgrade to my amp.