Hi Whitebread,
The frequency response is much improved, as is the power output (especially for lower impedance headphones - under 100 ohms) by the removal of the 22 ohm resistors on the output stage.
At the last NY Headfi Meet, I heard the Senn Momentums with the RWAK100 and can confirm that they sounded excellent together.
I own the HD650s (and LC2 and 3s) and they also work very well with the RWAK100. None of these headphones sound "blah" with the
RWAK100. Never heard the GR06s...
And I'm not here to make comparo's to the DX100 - but I can tell you straight up that after the RWAK100 mod, the AK100 is much better sounding throughout the whole frequency spectrum. Ask David Chesky of Chesky Recordings / HDtracks.com, and Srajan Ebaen of 6moons.
This thread has many RWAK100 owners, but it is long and there is a lot of posts to read through (almost 5,000!):
http://www.head-fi.org/t/630288/the-new-iriver-ak100-a-high-end-dap
All the best,
Vinnie